ADAMS. — TRANSMISSION OF llONTGEN HAYS. 675 



might be independent of transient changes in the external magnetic 

 field, a hollow cylinder of soft iron, with wall 0.6 cm. thick and with 

 the necessary apertures, was slipped over the case. The effects 

 believed to be due to external magnetic disturbances were thereby 

 much diminished. Changes in the room temperature promptly 

 produced a drift of the zero reading. To avoid this drift as much 

 as possible, the instrument was jacketed with cotton wool and hair 

 felt, the aluminium window for the rays being covered only by a 

 thin layer of the latter, which appeared by the use of the fluoro- 

 scope to be perfectly transparent. In addition to this, a double- 

 walled house of asbestos, large enough for a man to enter, w^as built 

 around the instrument, the partition, and the tube. The aperture 

 for observing the mirror was protected by a water-window about 8 cm. 

 thick, and the telescope and scale were placed outside the asbestos 

 house. These precautions were found sufficient for seasons w^hen there 

 was no artificial heat in the building, but during the winter the interior 

 of the house had to be kept at an approximately constant temperature 

 by means of an electric stove and an automatic regulator. Because of 

 the small mass of the suspended system — one or two tenths of a 

 gram — mechanical vibrations, such as those produced by machinery 

 in neighboring buildings or by a high wind, greatly limited the hours 

 available for work w'ith the instrument. 



The tube ^ employed as a source of Rontgen rays was one especially 

 designed for use with a Tesla coil. It is illustrated in section in 

 Figure 3. The Tesla coil was operated with power obtained indirectly 

 from the incandescent lighting mains. From those mains the alter- 

 nating current (110 volts, 60 cycles per second) was led to the pri- 

 mary of a step-up transformer and an adjustable resistance in series 

 with it. The current from the secondary of this transformer charged a 

 condenser of 0.005 mfd. capacity to a difference of potential of from 

 5,000 to 10,000 volts. The condenser was connected in series with the 

 primary of the Tesla coil and a Cooper-Hewitt interrupter, and dis- 

 charged itself through that circuit with a frequency of about 7 X 10^ 

 oscillations per second. Under these circumstances a spark from the 

 Tesla secondary 33 cm. long between points was obtained ; but for ordi- 

 nary purposes a spark of half that length proved sufficient. By keeping 

 the interrupter at a constant temperature of from 75° to 90° C, it was 

 found possible to maintain a remarkably uniform activity of the tube 

 for considerable periods, especially in the case of new tubes. 



8 Manufactured by the Macalaster Wiggin Company, Boston. 



