Dr. W. IT. Eccles on Coherers. 873 



condensers of the Marconi pattern, K is an air condenser, 

 L and 1/ are coils wound on waxed cardboard tubes. The 

 six-contact key S is designed to throw the telephone T quickly 

 from the detector circuit to the direct current circuit. In the 

 position of S shown in the diagram the telephone is in series 

 with the detector and part of the potential-divider P 2 , and at 

 the same time the primary inductance L/ is connected through 

 portion of the potential-divider P x and the interrupter I. 

 When the key S is depressed the telephone is connected 

 directly through the intenuptor I and the potential-divider P s , 

 and the secondary circuits are disconnected. The mutual 

 inductance between primary and secondary can be varied by 

 running 1/ along a straight slipway to or from the coil L ; 

 but usually the coils were held so that their mutual inductance 

 was a few hundreds of centimetres. The resistance r is an 

 important detail. Its purpose is to prevent the formation of 

 oscillations in L/ at the make of the primary current; calcu- 

 lation (as well as experiment) shows that if r lies within 

 certain limits oscillations occur in L' at the break only. The 

 amplitude of the oscillations is governed by aid of the potential- 

 divider Pj. The frequency of the interruptor I was chosen of 

 the low value 22 per second ; thus resonance with the telephone 

 diaphragm was avoided, and the balancing of the sound pro- 

 duced by the detector with that produced by P 3 facilitated. 

 The electrical dimensions of the apparatus were as follows : — 



1/ = 33,700 cm., C = 5020 cm., L =■• 42,700 cm., 



C = 3940cm., K = 1210 cm., r = 20 ohms, T = 311 ohms. 



The mutual inductance between Land 1/ ranged from 200 to 

 2500 cm. in obtaining the results described below. These 

 sizes were settled upon after many changes, partly because 

 they were practical sizes (wave-length about 820 metres), but 

 chieHy because certain later calculations were made easier. 



Details of Method. 



In the experiments the primary and secondary were used 

 slightly out of tune. This is very necessary when detectors 

 whose resistance varies are under study ; for the variations 

 of a coherer's resistance are quite wide enough to carry the 

 secondary circuit erratically from one side to the other of 

 perfect resonance if the circuits are initially very near that 

 condition, with the result that inconsistent measurements 

 would be inevitable. It is, of course, as easy to deduce 

 formulae for the mistimed as for the tuned circuits. The 



Phil. Mag. S. 6. Vol. ID. No. 114. June 1910. 3 L 



