380 Prof. J. Trowbridge on the Elect 



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the Plante rheostatic machine. This apparatus, I think, has 

 not received sufficient attention from physicists. In connexion 

 with a large battery it is very efficient and it enables one to 

 form an estimate of the high electromotive force that one 

 employs in the study of the Bontgen rays. I have slightly 

 modified the form of the machine as it is given by Plante. 

 The main principle consists in charging leyden-jars in 

 multiple and then discharging them in series. The proportion 

 of the length of spark to the number of jars is very close. 

 Knowing the electromotive force of the battery which charges 

 the jars we can estimate the voltage necessary to produce 

 sparks of different lengths. I speedily found that at least 

 one hundred thousand volts were necessary to produce the 

 Bontgen rays, and they were produced more intensely as I 

 increased the voltage, certainly to the point of five thousand 

 volts. 



In order to ascertain whether the discharges through the 

 Crookes tubes when the Bontgen rays were apparently pro- 

 duced most strongly were oscillatory, I first placed a Geissler 

 tube in the circuit with the Crookes tube and carefully 

 observed the appearances of the two electrodes of the Geissler 

 tube. They were quite alike and indicated an oscillatory 

 discharge. I then replaced the latter tube by a small spark- 

 gap and photographed the spark in a rapidly revolving mirror. 

 The photograph showed at least ten oscillations with a period 

 of about one millionth of a second with the Crookes tube and 

 the circuit I employed. Furthermore, applying the method 

 of estimating resistances by the method of damping, I found 

 that the resistance of the rarefied medium was less than five 

 ohms. The energy, therefore, at the moment of the emission 

 of the Bontgen rays was not far from three million horse- 

 power acting for one millionth of a second. I employed also 

 a Crookes tube with an aluminium mirror of about two centi- 

 metres focus. The resistance of this tube to the discharge 

 was the same as that in which the mirror bad a focal length 

 of six centimetres. Incidentally, there seems to be no 

 advantage in shortening the distance between the kathode 

 and the anode by employing a mirror of short focus. Struck 

 by the fact that the distance between the electrodes did not 

 appear to make any appreciable difference in the resistance of 

 the Crookes tube, I replaced the latter by a spark-gap of six 

 inches in length in air and photographed the spark in another 

 gap in air in the* same- circuit. This latter gap was one 

 quarter of an inch. The photographs showed on an average 

 the same number of oscillations whether the secondary spark- 

 gap was six inches in length or one inch in length. I found 



