ELECTRIC AND HEAT WAVES NICHOLS AND TEAR 



179 



are omitted from the diagram. The wave lengths sent out by the 

 oscillator depend on the length and diameter of the tungsten cylin- 

 ders, H x and H 2 . For the shortest waves generated, these cylinders 

 were 0.1 mm. long by 0.1 mm. diameter (0.004 by 0.004 in.). In 

 use these small tungsten cylinders are rapidly worn away by the spark. 

 For single observations, therefore, the oscillator was in most in- 

 stances operated for periods of less than half a second. 



The receiver. — The type of electric wave receiver employed was 

 a newly designed modification of the Nichols radiometer, which 

 makes use of the curious temperature effect in rarified gases dis- 



Fig. 1. — Schematic sketch of Hertzian oscillator unit 



covered 50 years ago by Sir William Crookes. He found that if 

 a thin plate, warmer on one face than on the other, is mounted 

 in an inclosure from which the air has been pumped out until only 

 about 0.0015 part remains, the residual gas exerts a very slight ex- 

 cess pressure on the warmer side of the plate. To illustrate this 

 effect, Crookes designed the little radiometers or light mills often 

 seen revolving in optician's windows. 



To utilize this effect in a very sensitive electric wave receiver, the 

 instrument shown in the cross-sectional diagram, Figure 2, was con- 

 structed. A greatly enlarged diagram of the suspended receiver 

 system is shown at A. D is a very thin whip of drawn glass bear- 

 ing two similar cross arms, EE. To these are attached two very 



