December 9, 1909J 



NA TURE 



169 



Walter's Tantalum Detector. 



be inserted in the receiving circuit to enable us to receive 

 a continuous sound in the telephone, which can be resolved 

 into Morse dot and dash signals by the key in the trans- 

 mitter. The operator usually wears on his head a double 

 telephone, and listens to these long and short sounds in 

 the telephone, and writes down each letter or word as he 

 hears it. 



The reception of signals in modern radio-telegraphy is 

 most usually effected by ear by means of some type of 

 oscillation detector cap- 

 OS volt ^ble of actuating a 

 i^t " telephone. It is im- 

 ' .' portant, then, to notice 

 that, to obtain the 

 highest sensitiveness 

 when using the tele- 

 phonic method of re- 

 ception, the spark fre- 

 quency or number of 

 oscillation trains or the 

 number of interruptions 

 of the persistent train 

 per second must take 

 place at such a rate 

 that it agrees with the 

 natural time period of 

 ^ the diaphragm of the 

 telephone 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 1000. 

 Thus Lord Rayleigh {see Phil. Mag., vol. xxxviii., 1894, 

 p. 285) measured the alternating current in microamperes 

 required to produce the least audible sound in a telephone 

 receiver of 70 ohms resistance at various frequencies, and 

 found values as follows : — 



Table IL 



Frequency 128 192 236 307 320 384 512 640 76S 



^m'micp'X'res'"'} =^ ='5 o'^S °-49 "'S- o'-S o'o; 0-04 o'. 

 M. Wien found for a Siemens telephone somewhat 

 different results, viz. : — 

 Frequency 64 128 256 512 720 1927 1500 



Lea^t audible current) . q .,- .^ .„^, 



f j2 15 013 0027 0000 0013 0024 

 in nncroamperes ...) j j / 



Both, however, agree in showing a maximum sensitive- 

 ness 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 radio-telegraphic stations are now designed to 

 give currents with a frequency of about 300 or 600 alterna- 

 tions per second, so that, when producing discharges of a 

 condenser, the number of sparks per second may be at 

 least 600, and fulfil the conditions for giving maximum 



Afplu-d I 'o/tagc. + 



Fig. 16. — Characteristic Curves of Carborundum Crystal. 



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 conduc- 

 tivity 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 .^rmy, in 1906, 

 that a mass of carborundum, which is a crystalline carbide 

 NO. 2093, VOL. 82] 



of silicon formed in electric furnaces, can act as a detector 

 of electric oscillations if inserted in the circuit of an 

 antenna, the crystal mass being held strongly pressed 

 between two spring clips, which are also connected by a 

 sliunted 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 in- 

 vestigated by Prof. G. W. Pierce, of Harvard, and he 

 showed that a single crystal of carborundum has remark- 

 able unilateral conductivity for certain voltages when held 

 with a certain contact pressure between metallic clips. 

 Thus for a crystal held with a pressure of i kilogram, and 

 subjected to an electromotive force of 30 volts, the con- 

 ductivity in one direction through the crystal was 4000 

 greater than in the opposite direction (Fig. 16). The result 

 of these experiments was also to show that the current 

 voltage curve or characteristic curve of a carborundum 

 crystal is not linear — that is to say, the crystal, as a con- 

 ductor, does not comply with Ohm's law, for the resist- 

 ance of the crystal decreases as the current is increased. 

 Hence the conductivity of the crystal is a function of the 

 voltage acting on it (Fig. 17). Accordingly, if we pass a 

 current from a local cell through a crystal under a voltage, 

 say, of 2 volts, a telephone 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 

 25 and 1-5 volts, but the corresponding currents would 

 be, say, 8-4 and i-8 microamperes, as shown by an experi- 

 ment with one particular crystal employed by Prof. Pierce. 

 The mean current would then be 5-1 microamperes, whereas 



-^14 18 

 2 I ' 



AfpliCii l'oll<igc 



the steady voltage of 2 volts would only pass a current 

 of 4 microamperes. Hence, apart from the unilateral con- 

 ductivity, 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 radio-telegraphic 

 detector. It consists of a conglomeration of crystals 

 arranged in a disorderly manner, or not so symmetrically 

 as to neutralise one another's unilateral conductivity. 

 Hence the mass of crystals, like 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 convert them into short unidirectional trains which 

 can affect a galvanometer or telephone ; or, secondly, we 

 may place the crystal between the ends of a circuit con- 

 taining 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 

 the steady voltage of the local cell. The telephone, there- 

 fore, detects this change in the average value of the 

 current by a sound emitted by it. Prof. Pierce has dis- 

 covered that several other crystals possess similar prtjper- 

 ties to carborundum — for exainple, hessite, which is a 

 native crystalline telluride of silver or gold ; an anatase. 

 which is an oxide of titanium ; and molybdenite, which is 



