Charge sniffer for electrostatics 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 electricity is described. It operates as an electric charge 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, electrostatic induction, 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 voltage 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. Electrometers 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.