176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
used. An ordinary telephone receiver is most sensitive, according to 
the researches of Lord Rayleigh and M. Wien, for some frequency 
lying between 500 and 1,000. Thus Lord Rayleigh (see Phil. Mag., 
vol. 38, 1894, p. 285) measured the alternating current in micro- 
amperes required to produce the least audible sound in a telephone 
receiver of 70 ohms resistance at various frequencies, and found 
values as follows: 
TABLE II. 
HrequenGypencnssocceoaesmceesieeceeesceeesece 128 | 192] 256] 307] 3820] 384] 512] 640 768 
Least audible current in microamperes...... 28} 2.5 | 0.83 | 0.49 | 0.32 | 0.15 | 0.07 | 0.04 0.1 
M. Wien found for a Siemens telephone somewhat different results 
Viz: 
128 
1.5 
256 
0.13 
512 
0. 027 
720 
0. 008 
1, 927 
0.018 
1,500 
0. 024 
INFEQUCNCY? oc eis ce aaioatesete secret Sseec ed acwsiesesecioess 64 
Least audible current in microamperes.............----.- 
Both, however, agree in showing a maximum sensitiveness for 
currents of a frequency between 600 and 700. This is due to the 
fact that the frequency of the actuating current then agrees with the 
natural frequency of the ordinary telephone diaphragm. Hence, 
alternators for large power radiotelegraphic stations are now designed 
to give currents with a frequency of about 300 or 600 alternations per 
second, so that, when producing discharges of a condenser, the num- 
ber of sparks per second may be at least 600, and fulfill 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 pos- 
session 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 1906, 
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 
telephone. This property of carborundum has been carefully investi- 
gated by Prof. G. W. Pierce, of Harvard, and he showed that a 
single crystal of carborundum has remarkable unilateral conductivity 
for certain voltages when held with a certain contact pressure be- 
tween metallic clips. Thus for a crystal held with a pressure of 
1 kilogram, and subjected to an electromotive force of 30 volts, the 
conductivity in one direction through the crystal was 4,000 greater 
