Maximum Frequency of X Rays from a Coolidge Tube, 343 



order to prevent electrostatic disturbances and electrical 

 losses, the galvanometer was placed on a rounded metal base, 

 insulated on paraffin blocks and covered with a round metal 

 screen, BB. The resistance in series in the galvanometer 

 was about 5000 megohms, and was such that only about 

 30 per cent, of the total current from the machine passed 

 through it even at the highest voltage employed, viz. 

 115,000 volts. 



It was at first intended to measure the voltage across the 

 tube directly by determination of the current through the 

 known high resistance. On account of the charging up of 

 the xylol tubes after the current had been passed for some 

 time, it was not found possible to determine with the 

 requisite accuracy the resistance of the xylol tube under 

 the conditions of temperature etc. when in actual use. 



The deflexions of the galvanometer were instead standard- 

 ized by the spark method. For this purpose, we made use 

 of a spark-gap composed of large copper spheres, SS, 

 20 "5 cm. in diameter, constructed by Dr. Makower some 

 years ago. Over the range of voltage employed, the voltage 

 required to produce a spark was practically the same as for 

 parallel plates. The tables employed were those given by 

 C. Miiller, Ann. d. Phy*. xxviii. p. 612 (1907). 



The method of standardization was as follows : — The 

 heating current was adjusted to the required value and the 

 machine was run for five or six minutes at about the voltage 

 required, so that the resistance of the xylol tubes should 

 reach a steady state. The length of the spark-gap was then 

 carefully adjusted, and a number of! observations made of 

 the deflexion of the galvanometer at the moment the spark 

 passed, care being taken that the voltage rose slowly to the 

 sparking-point. In a similar way, observations were made 

 at the end of a series of experiments, but no certain change 

 in the deflexion was ever observed over the interval of a few 

 hours. A gradual increase of the xylol resistance was, 

 however, observed over the interval of several months re- 

 quired for the experiments. The deflexion of the galvano- 

 meter was found to be nearly proportional to the voltage 

 over a considerable range. For voltages greater than 

 30,000, the spark-gap method was very suitable for cali- 

 brating the galvanometer directly. For voltages below this, 

 it was found more convenient and accurate to take the 

 deflexion of the galvanometer as proportional to the voltage. 

 The correctness of this was confirmed on several occasions 

 by determination of the deflexion of the galvanometer for 

 a voltage read directly on a Kelvin astatic voltmeter. 



