674 



Professor J. A. Fleming 



[June 4, 



second, so that, when prodnciug discharges of a condenser, the 

 number of sparks per second may be at least 600, and fulfil the con- 

 ditions for giving maximum sound in the telephone of the receiver 

 per microampere. Another class of oscillation detector recently 

 discovered comprises the crystal detectors which depend on the 

 possession by certain crystals of the curious property of acting as an 

 electrical valve, or having greater conductivity in one direction than 

 the other, and also on not obeying Ohm's law as conductors. It was 

 discovered by General Dunwoody of the United States Army, in 11)06, 

 that a mass of carborundum, which is a crystalline carbide of silicon 

 formed in electric furnaces, can act as a detector of electric oscilla- 

 tions if inserted in the circuit of an antenna, the crystal mass being 

 held strongly pressed between two spring clips, which are also con- 

 nected by a shunted voltaic cell in series with a telephone. When 

 feeble oscillations are set up in the antenna, a sound is heard in the 



15 



ir. 



10 5 n m 



- Applied Voltage. + 



Fig. 16. — Characteeistic Curves of Carborundum Crystal. 



telephone. This property of carborundum has been carefully investi- 

 gated by Professor U. W. Pierce of Harvard, and he showed that 

 a single crystal of carborundum has remarkable unilateral conduct- 

 ivity for certain voltages when held with a certain contact pressure 

 between metallic clips. Thus for a crystal held with a pressure of 

 1 kilogram, and subjected to an electromotive force of oO volts, the 

 conductivity in one direction through the crystal was 4000 greater 

 than in the opposite direction (see Fig. 16). The result of these 

 experiments was also to show that the current \oltage curve or 

 characteristic curve of a carborundum crystal is not linear — that is to 

 say, the crystal as a conductor does not comply with Ohm's law, for 

 the resistance of the crystal decreases as the current is increased. 

 Hence the conductivity of the crystal is a function of the voltage 

 acting on it (see Fig. 16a). Accordingly, if we pass a current from a 

 local cell through a crystal under a voltage say of 2 volts, a telephone 



