190 SCIENTIFIC AMERICAN SUPPLEMENT No.2124 September 16, 1916 molten mass, and then an arc immediately formed. The position of the oxide, brought about by increasing tem­ the ,characteristic fall as steeply as possible. frequencies admission of a small quantity of oxygen also precipitated perature. I have found similar effects to a slight extent of over a million are easily reached. an arc. in nickel and cobalt. These results have a bearing on If the oscillations are weak, the direct current through Intermittent Dischrge. Corona.-Both glow and metal­ the use of the iron arc as a wave-length standard in spec­ the arc simply fluctuates. This can occur with the carbon lic arc, as well as the discharge from points, are some­ troscopy. arc. If the oscillations are stronger, they put the arc times found to be intermittent. For this to be the case, Glow-arc Oscillaiions.-The effect at the cathode to out at each cycle, and before the arc relights the con­ the current must be relatively small and the capacity of which reference has been made is of the nature of a denser becomes charged. The oscillating arc used in the external circuit considerable. There is usually a cer­ rapid pulsation back and forth between glow and arc. radio-telegraphy is of this type. A third type, in which tain critical external resistance at which the intermit­ To obtain it, the supply electromotive force must be the oscillations are powerful enough to make the arc tence vanishes, as shown by the cessation of sound in a high, and the gas should be hydrogen at atmospheric relight in the opposite direction, produce a coarse tone telephone connected to the circuit. The phenomenon is pressure, mixed with a little hydrocarbon vapor. The and are not much used. closely related to the use of the arc as a generator of electrodes may be of any metal, placed very close to­ Effect of Second Harmnnic.-When the oscillations electric oscillations. The Cooper-Hewitt mercury inter­ gether, the discharge current being of the order of 1/10 are of the second type, extinguishing the arc at each rupter is a notable example of intermittent a,rc. ampere. cycle, the wave form is very complex. The second har­ In a strong magnetic field an arc can also be made When the discharge is a glow, a spot on the cathode monic is especially prominent. As an example of oscilla­ intermittent. In this case the instability is due to the becomes heated to such a degree that a small arc forms, tions of this type of audible frequency, the following lengthening of the column of vapor by the magnet, caus­ but the voltage then drops so much and the current experiment may be described. ing the voltage to rise above the point where the dis­ increases so little, owing to the shape of the characteristic A carbon arc is made to sing by connecting a large nharge can be maintained. Owing to the incandescence curve, that the energy is too slight to maintain the arc, capacity and a self-inductance coil of low resistance in of the cathode, the discharge quickly relights and the and immediately the glow sets in again. Such pulsations parallel with it. A second coil is connected to a capacity operation repeats itself. between arc and glow are of a frequency anywhere be­ such that the natural period of the secondary circuit is The corona discharge from high voltage transmission tween 1 and 250,000, depending on various conditions. nearly the same as that of the circuit connected to the lines is similar to a glow discharge, and is said to be often I have succeeded in converting 66 per cent of the power arc. When the two coils are brought near together, the intermittent. It exhibits the characteristic cathode and applied to the discharge into high frequency oscillations pitch of the arc changes, owing to the effect of close anode phenomena, as well as the tendency to concentrate of this'type. Unfortunately, the nature of the discharge coupling. If the pitch falls, it can be made to reach an at certain points which were mentioned above. ,Since the is such that the conversion of large amounts of power by unstable value from which it suddenly jumps up a whole air serves as one electrode, the corona has in some re­ this means is impossible. octave, the discharge remaining stable at this higher spects more in common with the discharge from points In these experiments ,no capacity was connected in pitch. than with'1he typical glow. parallel to the discharge, though the adjacent parts of The explanation of this is, that when any two oscillat­ Rotation at the Anode.-Two effects will now be the circuit acted as capacities for the high frequency ing circuits are closely coupled there are two possible described, illustrating the instability of gas discharges currents. frequencies, one higher and one lower, in either of which The Singing Arc - as certain currents. The first of these has to do with . In the singing arc, including the oscillations may take plaee. At a certain stage of the anode, the second with the cathode. Poulsen arc used in radio-telegraphy, a capacity and coupling the higher frequency is just an octave above Tn the iron arc'in free air the discharge normally takes self-inductance in series are placed in parallel with the the lower. If now the arc is on the lower frequency place between globules of molten oxide. The second arc. The effect of the capacity is to drain energy from and has a very strong second harmonic, then if resonance stage sets in at a little over an ampere. A close exam­ 'the arc when the current in the condenser circuit is in is also possible at the octave above, the harmonic may ination shows that the bright spot at the positive end of one direction, and to strengthen the arc current when in be reinforced to such an extent as to assume the role of the arc is in rapid rotation, describIng a minute ring. the other direction. Continuous oscillations are thus fundamental frequency its9lf. The pitch then rises to As the current increases, this form of rotation suddenly maintained, the condenser circuit receiving energy and the higher octave. changes to one in which the ring is still smaller, then at timing its own impulses in a manner analogous to the It is hoped that the phenomena that have been de­ about 1.8 amperes a large ring takes its place, which balance wheel or pendulum of a clock. scribed may serve to illustrate the unstable nature of presently yields the stage to a smaller ring again. The point of chief interest here is that, as Kaufmann, some form of gas discharge, as well as the very diverse Each of the four types of rotation has its characteristic Simon and others have shown, such oscillations are ways in which the instability asserts itself. diameter and speed, and produces a faint note of a cer­ possible only when the arc (or glow) possesses a falling With respect to practical conclusions, the following tain pitch. Oscillograph records show that the current characteristic. Simon has found that the alternating­ may be said: for stability, an arc shonld be operated and voltage undergo corresponding changes. Reasons current, or "dynamic," characteristic differs from the with snpply electromotive force, external resistance, and for believing that these move­ direct-current, or "static," characteristic, the difference current all large. Or, if the discharge is to be a glow, a have been giventi elsewhere3 ments are d c to a reduction of the oxide at the positive being due to a sort of hysteresis, and that the result of region on the characteristic should be chosen at a safe base of the discharge, causing the base to wander to this is that at high frequency the dynamic characteristic distance from a critical point, having due regard to the regions richer in oxygen. The changes from one type tends to change from a falling to a rising type. This supply electromotive force. For oscillations, a steeply the to the next are due to modifications in com- liniits the attainable frequency, but by reducing the falling characteristic, small current, large electromotive 'Phys, Rev. 2, p. 249, 1913. hysteresis and choosing such conditions as will make force and artificial cooling of the electrodes are f(l,ctors. The History of the Safety Lamp* The Result of One Hundred Years of Effort By Prof. F. W. Hardwick, M.A., and Prof. L. T. O'Shea, M.Sc. TIlE earliest references to the nature of fire-damp ap­ down the shaft by mirrors, and proposals were made at the objects for which it was formed, and it was due to pear to occur in the latter part of the seventeenth cen­ a later date to nflect the light into the workings. its efforts that the safety-lamp was introduced at this tury. From these it is evident that the risk of its igni­ Some progress was also made in respect of the knowl­ period. Among those who, at the time when the society tion and explosion were known, and it was recognized edge of the properties and composition of fire-damp. was formed, were actively engaged in endeavoring to that ventilation (to keep the air very "quick") was a Some investigators recognized its similarity to hydrogen discover some safe means of lighting coal-mines were means of prevention of these dangers. The recognition gas, and were aware that it was heavier than hydrogen, William Reid Clanny and George Stephenson, and the of the presence of fire-damp by means of the candle­ but lighter than air, and although Volta pointed out the society, apparently influenced by Mr.