1909] 



on Researches in Radiotelegraphy . 



675 



being inserted in series with the cell, and if we apply an oscillatory 

 voltage also to the crystal, which varies say between +0'5 and 

 — 0*5 volt, then the crystal is alternately subjected to a voltage of 

 2*5 and 1*5 volts, but the corresponding currents would be say 

 8'4 and 1*8 microamperes, as shown by an experiment with one 

 particular crystal employed by Professor Pierce. The mean current 

 would then be 5*1 microamperes, whereas the steady voltage of 

 2 volts would only pass a current of 4 microamperes. Hence, apart 

 from the unilateral conductivity, and merely in virtue of the fact 

 that the characteristic curve is not a straight line, we find that such 

 a crystal or even a confused mass of crystals can act as a radio- 

 telegraphic detector. There are, therefore, two ways in which a 

 crystalline mass of carborundum can be used as a radiotelegraphic 

 detector. It consists of a conglomeration of crystals arranged in a 



Applied Voltage. 



Fig. 16a. 



disorderly manner, or not so symmetrically as to neutralise one 

 another's unilateral conductivity. Hence the mass of crystals, Hke 

 the single crystal, possesses unilateral conductivity, and also a 

 conductivity which is a function of the voltage applied to it. We 

 may then use it without a local cell, and avail ourselves of its valve 

 property to rectify the trains of oscillations in the antenna and con- 

 vert them into short unidirectional trains which can affect a galvano- 

 meter or telephone ; or secondly, we may place the crystal between 

 the ends of a circuit containing a telephone and a shunted voltaic 

 cell, and then on passing oscillations through the crystal we hear 

 sounds in the telephone due to the fact that the conductivity is a 

 function of the voltage, and is therefore increased more by the 

 addition than it is diminished by the subtraction of the electromotive 

 force of the oscillations to or from th{!: steady voltage of the local 

 cell. The telephone, therefore, detects this change in the average 

 value of the current by a sound emitted by it. Professor Pierce has 

 discovered that several other crystals possess similar properties to 



