580 WHEELER P. DAVEY 



APPARATUS 



X-rays were produced by a water-cooled Coolidge tube (tung- 

 sten target) operating directly from a high-tension 60 cycle 

 transformer. Such tubes will rectify their own current up to 

 50 to 100 miUiamperes, at 50 kilovolts (R. M. S.).=' Oscillo- 

 graph tests showed that the transformer was of such a type 

 that when operated under the above conditions, the wave-form 

 of the secondary (high voltage) resembled that of the primary 

 (low voltage), and the inverse voltage did not exceed the direct 

 voltage (i.e., operating voltage of the tube) by 5 per cent. Fur- 

 ther tests with the oscillograph showed that the R. M. S. volt- 

 age of the secondary differed from that shown by a voltmeter 

 coil by not more than 3 per cent. Tube voltage was therefore 

 measured in terms of R. M. S. kilovolts, as shown by the meter. 



The voltage impressed upon the primary was controlled by 

 means of an auto-transformer of such size as to cause no appreci- 

 able change in wave-form. The wave-form used was very nearly 

 sinusoidal. 



The filament of the X-ray tube was heated by current from a 

 small transformer. This was connected through a ballast trans- 

 former to the terminals of the circuit supplying the auto-trans- 

 former. Connections are shown in figure 1. 



Tube current was measured with a direct-current milliam- 

 meter. Since this current was pulsating (half of every wave 

 being suppressed by the rectifying action of the X-ray tube), 

 oscillograph records were taken to compare the meter-reading 

 with the instantaneous value of the current. It was found in all 

 cases that the value of the mean current as shown on the meter 

 was almost exactly half the value of the peak of the wave. 

 The current through the tube was therefore read in mean mil- 

 liamperes on the meter. The wave-form of this current was simi- 

 lar in every way to that of other Coolidge tubes. 



The X-ray tube was in a lead box whose walls were j inch 

 thick. This provided a safe protection from X-rays at the volt- 



" R. M. S. voltage is measured as the "root mean square" of the voltage 

 wave. This is in accordance with ordinary electrical practice in measuring 

 alternating voltages. 



