352 Messrs. Trowbridge and Richards on the Temperature 



the light of the spark between two cadmium electrodes in the 

 same circuit is quite bright enough for the purpose, and of 

 course any oscillations which crossed the spark-gap must also 

 go through the tube. Our next step, therefore, was to make 

 a series of such photographs of a spark discharged through 

 hydrogen, at first when the gas glowed with a white light 

 and snowed its many-line spectrum, and afterwards when it 

 exhibited the characteristic red tint and a spectrum of only 

 four lines in the visible portion of the spectrum. 



In order to obtain the white light in the hydrogen tube, it 

 was necessary to increase either the impedance or the resistance 

 in the circuit containing the tube. With a definite very small 

 amount of impedance we increased the resistance until the red 

 glow disappeared in the tube, and discovered on developing 

 the photographs which were obtained by means of the revolv- 

 ing mirror that the discharge was non-oscillatory. When, 

 however, the resistance in the condenser circuit was diminished, 

 the red glow began to appear, and the photographs taken 

 when all the resistance except the tube itself was removed 

 showed that the discharge was oscillatory. This also was 

 evident from the peculiar crackle of the spark, which Hertz 

 remarked was essential in performing his experiments on 

 electric waves. The apparatus used in this and subsequent 

 experiments is sketched in the accompanying diagram (fig. 1). 

 An examination of our photographs showed the interesting 

 fact that there were in general not more than two or three 

 complete oscillations ; the remaining ones which could have 

 been obtained from the given capacity and self-induction 

 having been damped by the resistance of the gas. The 

 question immediately arose, What is the resistance of the 

 gas at the instant of the discharge? For if an idea of this 

 can be obtained we can get an estimate of the amount of beat 

 developed in the gas during each oscillation. A Thomson 

 Electrostatic Voltmeter connected to the ends of the hydrogen 

 tube indicated a difference of potential of over 1800 volts, 

 and this difference of potential could only be obtained by 

 substituting for the Greissler tube a resistance of many 

 thousand ohms. The indications, however, of this instru- 

 ment in this case are of no value ; for we discovered that a 

 resistance of from ten to twenty ohms was sufficient to pro- 

 duce the same amount of damping which the gas exerted. 

 The resistance of the gas, therefore, could not be greater than 

 these amounts'*. It is evident, therefore, why the voltmeter 

 gives erroneous readings. On account of the inertia of the 

 moving parts, and the very short time of the discharge, it 

 * "Damping of Electrical Oscillations," Proc. Amer. Acad. 1891. 



