Charge sniffer for demonstrations ͒ Mihai P. Dincaa Faculty of Physics, University of Bucharest, P.O. Box MG-11, Bucharest-Magurele 077125, Romania ͑Received 15 April 2010; accepted 21 September 2010͒ An electronic electroscope with a special design for demonstrations and experiments on static is described. It operates as an sniffer by detecting slightly charged objects when they are brought to the front of its sensing electrode. The sniffer has the advantage of combining high directional sensitivity with a logarithmic bar display. It allows for the identification of electric charge polarity during charge separation by friction, peeling, , batteries, or secondary coils of power transformers. Other experiments in electrostatics, such as observing the electric field of an oscillating dipole and the distance dependence of the electric field generated by simple charge configurations, are also described. © 2011 American Association of Physics Teachers. ͓DOI: 10.1119/1.3531961͔

I. INTRODUCTION audible tone of constant amplitude with the frequency related to the reading. That is, an upward change in the The leaf electroscope and its variants are well suited for frequency of the audible tone indicates positive charge, and a use as charge detectors and as crude electrostatic voltmeters downward change indicates negative charge. This change of for demonstrations, especially when equipped with a projec- the tonal voltmeter is more impressive and easy to observe tion system. The advantage of the leaf electroscope is its during demonstrations, but suffers from the same limitations simple construction, and its operation, based on the repulsion as those based on brightness. of like charges, is readily understood. When using the clas- This paper describes a new electronic electroscope that has sical electroscope for detecting charge polarity, a reference been designed primarily for demonstrations and experiments. charge with known polarity is required. This procedure in- The electroscope is designed for use as a “charge sniffer” volves additional reasoning based on the cumulative effect of and is able to instantaneously detect the charge of objects in charges with different polarities. The traditional electroscope front of its nose and simultaneously provide a measure of the also has other limitations related to its modest sensitivity, magnitude of the charge. In conjunction with a Faraday cup which often restricts the experiments to cold winter days and a digital voltmeter, it can also be used as a Coulomb when the humidity is low. meter. equipped with dc amplifiers are major com- The special design of its probe makes the detector sensi- petitors for demonstrations and classroom experiments.1–3 tive within a narrow angle around the normal of the probing They do not have the same disadvantages as electroscopes disk. The display indicator drops to half when the detected and low-cost versions are available. Many designs for elec- charge is placed at the same distance, but on a line inclined tronic electroscopes have been published,4,5 including those by 40° with respect to the normal. A piece of adhesive tape based on the field effect transistor6 and the integrated opera- charged by peeling can be detected from half a meter away tional amplifier.7,8 The amplifiers easily exceed the sensitiv- when placed on the line of maximum sensitivity. The probe’s ity limits of classical electroscopes, instantly discriminate the high sensitivity allows for the detection of charges separated charge polarity without a need for a reference charge, and by a pile of two or three 9 V batteries and makes it possible allow simple interfacing with different types of displays by to study the surface charge associated with steady currents providing an analog voltage directly proportional to the de- by using a low voltage power supply. The display consists of tected charge. Commercial versions are available.9–15 Sci- two LED bar graphs and is highly visible. Two different ence projects involving the construction of electronic elec- color LEDs are used to indicate the charge polarity, and the troscopes have become very popular,16–20 probably due to the magnitude of the induced charge is related to the number of extremely high sensitivity obtained with simple circuits and LEDs that are lit. A wide dynamic range is achieved by using low-cost displays using LEDs. a logarithmic scale. This scale also allows for a simple and A simple FET based tester screwdriver, intended to dis- rapid check of the induced charge dependence on distance. cover live wires without actual contact, was recently Changes in the electric field, such as those produced by os- proposed21 as a teaching aid in introducing concepts of elec- cillating dipoles, can also be observed due to the fast re- trostatics and circuits. sponse of the display. Resolution and reading accuracy are key features of the displays of instrumentation used in the laboratory. High vis- II. CHARGE AMPLIFIER ibility and ease of interpretation are most important for the displays of devices used in demonstrations and classroom The electronic electroscope ͑see Fig. 1͒ is housed in a experiments. Despite the apparent diversity of display types, grounded metallic box, with the electrostatic shield extended all but one that we have cited fall into two categories: Dis- bya5cmlong copper tube through which the input coaxial plays generally used in scientific instrumentation1–3,9–11,14,17 cable is passed and then connected to a BNC jack mounted at and crude indicators by brightness of a LED or incandescent the tube end. Two operation modes are possible for the am- lamp13,16,19,20 that do not allow for any record or comparison. plifier, namely, a Coulomb meter using a Faraday pail and a The single exception is the tonal voltmeter,22 which uses an charge sniffer. In the latter mode, a sensing electrode ͑shown

217 Am. J. Phys. 79 ͑2͒, February 2011 http://aapt.org/ajp © 2011 American Association of Physics Teachers 217 a more frequently used noninverting amplifier, only a frac- tion of the charge on the pail is transferred to the measuring capacitor and then converted to an output voltage. Conse- quently, for accurate measurements to be possible, the pail capacitance should be negligible compared to the input ca- pacitance of the amplifier. With the inverting configuration Fig. 1. The charge sniffer ͑with the sensing electrode shown detached͒. shown in Fig. 2, the input practically operates as a virtual ground and the pail charge is fully transferred to the feed- back capacitor even if the pail capacitance is not negligible ͑ ͒ compared to the feedback capacitance the equivalent input detached in Fig. 1 has to be inserted into the BNC jack to capacitance is the feedback capacitance multiplied by the form the sniffer nose. This electrode is made of a small disk, op-amp open loop gain and exceeds 30 ␮F͒. Because the cut from a 0.5 mm thick copper sheet, attached by soldering circuit is grounded, the detector nose in the sniffer mode can to a piece of wire. The disk diameter was chosen to be be considered to be at zero potential, which makes it easier to smaller by 1 mm than the inner diameter of the BNC con- understand charge induction on the detector disk. nector to avoid a short-circuit to the ground, and the wire The price to be paid for these benefits is a continual drift length was adjusted to keep the disk 0.5 mm inside the BNC in the output voltage, which is produced by integrating the connector to prevent touching it accidentally. input bias current of the operational amplifier by the feed- The schematic of the apparatus consists of two blocks, a back capacitor. Thus, before starting an experiment, the feed- charge amplifier, which converts the electric charge into a back capacitor has to be appropriately discharged by closing voltage level, and a display block. In Fig. 2, the amplifier the reset switch Sw1 for a short time while keeping the de- circuit diagram is presented. The negative feedback, having a tector nose far away from any charged object. To keep the dc loop gain of over 3ϫ105, keeps the inverting input of the ͑ drift rate at an acceptable level, special cautions are taken to operational amplifier very close to the ground within a few minimize the leakage currents entering into node A on the microvolts for the working output levels͒, providing a virtual circuit diagram. Thus, as switches Sw1 and Sw2 we used ground at this point. Consequently, when an object carrying relays23 with a very high insulation resistance of 1013 ⍀.To the electric charge Q is brought near the nose, part of the avoid leakage currents on the printed circuit board, all the field lines will be intercepted by the sensing electrode and a ͑ Յ ͉ ͉Ͻ͉ ͉͒ pins connected to node A are passed through 2 mm diameter charge Qind QindQ 0; Qind Q will be induced, which is holes made in the printed circuit board and then soldered to directly proportional to the electric flux intercepted by the the cable central conductor by using a point-to-point up-in- disk. Because the op-amp bias input currents are extremely the-air wiring technique.24 low, a charge −Qind will be forced on the left armature of C1, As will be shown in Sec. III, the display scale extends to / causing a voltage output V=Qind C1 to appear. If a lower 1.25 V without any further amplification and allows the de- sensitivity is desired, the feedback capacitance can be in- tection of a charge producing 60 mV at the amplifier output. creased by closing the switch Sw2. For the capacitor values When using a feedback capacitor of C1 =100 pF, full scale in Fig. 2, the sensitivity can be lowered 100 times. The entire corresponds to a charge of 125 pC induced on the sensing apparatus is powered by two 9 V batteries, and the supply electrode and the smallest induced charge that can be de- for the op-amp are obtained using Zener diodes. tected is 6 pC. Thus, the sniffer is at least 100 times more When a Faraday pail is connected to the BNC jack instead sensitive than the commercial versions we have cited, which of the sensing electrode, and a charged object is inserted into use a large 10 nF conversion capacitance to assure a proper ͑ ͒ the pail without touching it , almost all the field lines will be charge transfer from the ice pail capacitance. As a conse- intercepted by the pail walls and the induced charge will be quence, the amplifier described here is more sensitive to in- / −Q, which produces a voltage output V=−Q C1. Therefore, terferences caused by main supply wires by capacitive cou- the apparatus can also be used as a Coulomb meter. The fact pling, requiring a shielded ice pail and carefully shielded that the amplifier input is a virtual ground provides three connections, as well. When in sniffer mode, the real sensi- important advantages in comparison to the noninverting con- tivity is less than we calculated because only a part of the figuration used in almost all other electrometers. Because the electric flux is intercepted by the sensing electrode and, con- input has practically the same potential as the cable shield, sequently, the induced charge is less than the charge to be the leakage current in the cable is drastically re- detected. By using additional amplification ͑G=4͒ in the dis- duced. Also when a Faraday pail is connected to the input of play block, the sniffer can detect the charge on a 1 cm2 metallic plate charged at 10 V, when placed 1 mm away from its nose.

III. DISPLAY For demonstrations and classroom experiments, a simple, compact, and rugged apparatus able to quickly identify the polarity of the charge carried by an object and give a highly visible and easy to interpret indication about the charge mag- nitude when placed in the proximity of an object is often needed. Interfacing an to a personal computer allows for the design of excellent graphical interfaces, but the apparatus is neither simple nor compact. Digital displays Fig. 2. Circuit diagram of the charge amplifier. can be large but are slow and difficult to follow when the

218 Am. J. Phys., Vol. 79, No. 2, February 2011 Mihai P. Dinca 218 Fig. 3. Different configurations of the two display sections: ͑a͒ Side by side and ͑b͒ zero centered. display is changed. Their high resolution is appropriate for scientific instrumentation, but can cause confusion in simple demonstrations when the resolution exceeds the accuracy with which the experiment can be controlled. Needle meters that need special equipment for high visibility are slow and Fig. 4. Circuit diagram of the display block. not rugged enough. Moreover, an algebraic sign before the number or the direction of the needle does not emphasize the two different kinds of charges. disturbances. Resistor R3 sets the amplifier gain G=1 The LED bar graph display included in the apparatus de- / +R3 R2, and hence the display sensitivity. The red LEDs are scribed here has most of the desired features. As with the ͑ ͒ tonal voltmeter,22 the difference between positive and nega- driven by the integrated driver LM3915 U3 . Because the tive charges is not obvious. Using color has proven effective output provided by the electrometer amplifier is bipolar and in teaching electricity concepts25 and designing electric in- LM3915 accepts only positive voltages, the negative part is 26 struments for classroom demonstration. Therefore, I have clipped by diode D1. The thresholds are programmed by the used two separate display sections, each consisting of red voltage at pin 6, which is chosen to be 1.25 V. This level is and blue LEDs to discriminate between the two types of provided by the internal reference at pin 7 when the pin 8 is charges without using algebraic notation. Initially, the two grounded. Thus, the thresholds extend from 60 mV for LED sections can be placed vertically side by side ͓see Fig. 3͑a͔͒ 1 to 1.25 V for LED 10. To calculate the overall sensitivity of to reinforce the idea that we are detecting two different types the instrument, the gain has to be considered as well. The of charges. Then, we can investigate what happens if an un- LED current is set by resistor R8 according to / / ⍀ like charge of larger magnitude is brought near a detected ILED=12.5 V R8 +VREF 2.2 k ; the second term contrib- “red” charge. We will observe that the detector indication utes only 0.57 mA. By using the switch Sw1, the display can ͑ gradually decreases to zero and then increases, but this time be put in the bar graph mode for a more impressive appear- ͒ showing a “blue” charge. Students will observe that we can- ance or in the dot mode to extend the battery life. The blue not simultaneously have bright LEDs of both colors and then section is similar to the red one and will not be described in will discover that it is useful to place the two sections, as in detail. The only difference is that the amplified signal, after Fig. 3͑b͒ to mimic the number axis. The choice to be made being amplified by U1, is now inverted by op-amp U2 before ͑positive should be red or blue?͒ clearly emphasizes the ar- controlling the blue section display. Though drawn sepa- bitrariness of the convention chosen by Benjamin Franklin. rately for the simplicity, switches Sw1 and Sw2 are actually Each display section consists of ten LEDs separated by 5 the same device, and pins 9 from both LM3915 are tied mm, which can be driven either in a moving dot or bar con- together. If a ratio of 23.7/1 between the outmost thresholds figuration to indicate the charge magnitude. If desired, the is insufficient, the circuit can be changed by cascading two ͑ ͒ number of LEDs can be increased up to 20, with only slight LM3915 in each section see Ref. 27 . Thus, a dynamical changes in the electronics. If the display obeys a linear law, range up to 560/1 can be obtained, while the number of abrupt changes in indications are observed when a charge LEDs is either kept at ten by increasing the spacing to 6 dB ͑ ͒ approaches the probe due to the 1/r2 dependence of Cou- 2/1 or the number is doubled. lomb forces. When detecting a charge at a given distance, the dynamical range provided by the ten LED displays is only IV. EXPERIMENTS 10/1. Therefore, the LED thresholds form a geometric pro- gression with the ratio ͱ2. The progression obeys a logarith- By taking advantage of the high and directional sensitivity mic scale, and the thresholds of adjacent LED are separated of the charge sniffer, various experiments can be performed, by 3 dB. The logarithmic scale provides a less abrupt varia- such as charge separation by peeling, rubbing and hitting, tion during charge displacement and extends the dynamical superposition, charge separation by the piezoelectric effect, range to 23.7/1. Because the logarithmic scale is more suit- batteries and ac voltage sources, electrostatic induction in able for power law type dependencies, it is easy to check the metals and insulators, discrimination between insulators and 1/r and 1/r2 laws, as will be seen in the following. conductors, surface charge in dc circuits powered from bat- The circuit diagram for the display block is shown in Fig. teries, and shielding of static or slow varying electric fields.28 4. The signal from the charge amplifier block is first pro- For illustration, we describe a few of the possible experi- cessed by the noninverting amplifier built around U1. The ments. Because of the high sniffer sensitivity and small low-pass filter R1C1 hasa5Hzcut-off frequency and rejects amounts of charges involved in these experiments, special the 50/60 Hz interference signal and other high frequency cautions must be taken to avoid the interference caused by

219 Am. J. Phys., Vol. 79, No. 2, February 2011 Mihai P. Dinca 219 Fig. 7. The sniffer indication follows the oscillations of an electric dipole.

charged and the glass is positively charged. When the same cloth is used to rub a PVC pipe, it becomes positive and the PVC pipe becomes negative. Different materials can quickly be tested.

B. Electric field of simple charge configurations Although the case housing the sniffer and its ͑grounded͒ Fig. 5. Sniffer indication versus distance as calculated for an electrical field nose disturb the electric field produced by a pointlike or having ͑a͒ a1/r2 dependence and ͑b͒ a1/r dependence. cylindrical charge distribution, the deviation of the electric flux intercepted by the sniffer’s sensing electrode from the 1/r2 ͑or 1/r dependence is usually smaller than the sniffer’s the accumulation of charges on the table surface or on the measuring errors͒. Consequently, these dependences can be experimentalist’s clothes, a process that cannot be easily demonstrated with proper selection of the field sources. avoided. As a minimum, metallic foil should cover the table Pointlike charge. As mentioned, the thresholds of the and the experimenter’s clothes should be carefully chosen. LEDs form a geometric progression with the ratio ͱ2 Discharging good insulators can be tricky, but an open flame doubled each time the indication advances by two positions. 29 or a balanced bipolar ionizer can be very effective. Fora1/r dependence, the display value falls by two LED positions when the distance is doubled ͓see Fig. 5͑a͔͒, and / 2 A. Charge separation fora1 r dependence, the display value falls by four LED positions when the distance is doubled ͓see Fig. 5͑b͔͒.A Adhesive tape. A roll of adhesive tape is checked and small piece of silicone rubber tube is charged by friction and found to be initially electrically neutral. Then, a few centi- placed at such a distance to turn on LED 10. Then, the dis- meters long piece of tape is pulled out of the roll. When the tance is doubled and the display value falls by about four tape is brought close to the sniffer, it causes a strong red positions. Doubling the distance again makes the display indication, while the roll causes an equally strong blue indi- value decrease by about four additional positions. cation. Long cylinder. A 50 cm long piece of PVC pipe is charged The experiment can be repeated using two pieces of adhe- by rubbing and the same experiment is repeated. When the sive tape that were previously prepared by making nonsticky distance is doubled, the display value falls by about two handles, sticking them together ͑sticky side of one to the positions. nonsticky side of the other͒, and leaving them until the stray charge is neutralized. If the two pieces are quickly separated C. Electric dipole by peeling them apart, they become electrified with opposite polarities. Static dipole. Two small metallic balls are charged with Plastic bag. A plastic food storage bag is unrolled and then opposite polarities and placed on a glass plate. To explore the detached from the rest. The detached bag and the remaining dipole field, the plate is slowly rotated in front of the sniffer roll exhibit opposite charges. Each time a new bag is un- nose, as depicted in Fig. 6. The sniffer display shows red or rolled and detached it obtains the same type of charge. blue according to which charge is closer to it and no display A neutral glass rod is tapped against a neutral plastic rod value when the balls are symmetrically placed with respect or vice versa. The glass rod becomes positively charged, to it. while the plastic rod becomes negatively charged. Oscillating dipole. An electric dipole is formed using a If a cloth is used to rub a glass rod, the cloth is negatively fixed negative charged ball and a positive charged one sus- pended by an insulating wire ͑see Fig. 7͒. The magnitude of the charges is arranged to not give any display value when in the equilibrium position. Then, the pendulum is set in oscil- lation, and the sniffer display shows an oscillation in phase with the oscillations of the ball.

D. Charges separated by induction in metals Two metallic bars are grounded and then placed on glass insulators. Afterward, the bars are brought in contact, as shown in Fig. 8. The end B is set close to the sniffer nose, without touching it. When a positive charged object C ap- proaches the opposite end A, the red bar graph lights and Fig. 6. Exploring the dipole field by a slow rotation of the sniffer front. grows. Because the charged object C is too far for the sniffer

220 Am. J. Phys., Vol. 79, No. 2, February 2011 Mihai P. Dinca 220 2 J. M. Gregory, “Teaching electrostatics in schools,” Phys. Educ. 9 ͑6͒, 371–374 ͑1974͒. 3 J. Witzel, “Measuring Earth’s electrostatic charge,” IEEE Instrum. Meas. Mag. 5 ͑4͒, 46–47 ͑2002͒. 4 H. P. Stabler, “Inexpensive DC electrometer,” Am. J. Phys. 28 ͑7͒, xiii– xiv ͑1960͒. 5 Dean S. Edmonds, Jr., “An inexpensive vacuum-tube electrometer,” Am. J. Phys. 36 ͑11͒, 969–976 ͑1968͒. 6 S. W. Nelson and R. L. Howard, “Inexpensive electrometer amplifier,” Am. J. Phys. 34 ͑3͒, xxix–xxx ͑1966͒. Fig. 8. Electrostatic induction: The sniffer gives a positive indication as the 7 W. H. Jarvis, “A simple electrometer,” Phys. Educ. 24 ͑2͒,113͑1989͒. charged object C approaches the bar end A. 8 F. W. Inman and C. E. Miller, “An inexpensive electrometer for modern physics experiments,” Am. J. Phys. 40 ͑4͒, 623–624 ͑1972͒. 9 ͗www.leybold-didactic.de͘, search for 53214 or electrometer amplifier. nose to detect it, we can conclude that the end B is positively 10 Pasco, ͗www.pasco.com͘, search for CI-6555 or charge sensor. 11 charged. Removing object C causes the sniffer display value ͗www.vernier.com͘, search for CRG-BTA or charge sensor. 12 ͗ ͘ to go down. www.sciencelab.com , search for 10-501-12 or solid state electroscope. 13 ͗www.jupiterscientific.com.au/electroscope_polarity.jpeg͘. The previous experiment is repeated, but the two bars are 14 ͗www.jeulin.com/͘, search for Coulometer or 27201584. separated before object C is removed. Now, the sniffer shows 15 R. A. Morse, “Electrostatics with computer-interfaced charge sensors,” that the two bars acquired opposite charges as a result of Phys. Teach. 44 ͑8͒, 498–502 ͑2006͒. charge separation due to the influence of the charged object. 16 ͗www.eskimo.com/~billb/emotor/chargdet.html͘. If the two bars are brought into contact, the separated 17 ͗electroschematics.com/671/electroscope-measure-electrostatic-charge/͘. 18 R. Hull, “An experimenter’s electrometer,” Amateur Scientists Bull. 5 charges redistribute among the conductors and the bars re- ͑ ͒ ͑ ͒ ͗ turn to neutrality. 1 , 7–11 1998 , online at http://www.amasci.com/electrom/ sas51p1.html͘. Two neutral metallic proof-planes are kept in contact and 19 ͗www.pegna.com/page006.htm#9.%20ELETTROSCPIO͘. placed in front of a positively charged object, intercepting its 20 ͗www.feiradeciencias.com.br/sala11/11_53.asp͘. field. Then, they are separated and investigated by the sniffer. 21 S. Fischer and P. Gluck, “Tool teaches electricity concepts,” Phys. Educ. The sniffer shows that the plates acquired charge with oppo- 43 ͑3͒, 254–255 ͑2008͒. 22 ͗www.ece.rochester.edu/~jones/demos/tonal.html͘. site polarity such that the plate intercepting the field becomes 23 negatively charged, and the back side plate, positively SIL05-1A72-71L from Meder Electronic, provided by Farnell, ͗www.farnell.com͘. charged. 24 LMC6081 datasheet, National Semiconductor, ͗www.national.com͘, search for LMC6081. ACKNOWLEDGMENTS 25 T. Reeves, “Potential difference in colour,” Phys. Educ. 38 ͑3͒, 191–193 ͑2003͒. The author thanks Professor Petrica Cristea for valuable 26 M. Kamata and C. Hara, “An ammeter that indicates electric current by discussions on the proposed experiments and for carefully the movement of a light spot, and voltage by the colour,” Phys. Educ. 40 reading and commenting on the manuscript. ͑2͒, 155–159 ͑2005͒. 27 ͗www.national.com͘, search for LM3915. 28 A video of experiments performed with an earlier version of the charge ͒ a Electronic mail: [email protected] sniffer is available at ͗www.youtube.com/watch?vϭvAQ64Sv4iqY͘. 1 G. R. Davies, “An electronic gold leaf electroscope,” Phys. Educ. 9 ͑6͒, 29 A. Ohsawa, “Precisely balanced ionizer using atmospheric pressure glow 393–398 ͑1974͒. discharge in air,” J. Electrost. 63 ͑1͒, 45–57 ͑2005͒.

221 Am. J. Phys., Vol. 79, No. 2, February 2011 Mihai P. Dinca 221