Dr. W. H. Eccles on Coherers. 871 



published, yet only minute voltages nowadays arise in 

 practice. The purpose of! the present investigations is to 

 examine ihe action of a certain kind of coherer under " weak 

 signals," especially in respect of their energy relations. 



Method of Experiment. 



The self-restoring coherer to be examined was placed in a 

 circuit comprising the necessary source of steady electro- 

 motive force and a telephone receiver. At the same time it 

 was also connected in series with a condenser, to form, as in 

 practical wireless telegraphy, a shunt across the condenser of 

 an oscillatory circuit (fig. J ). This oscillatory circuit was 

 made a secondary circuit in relation to a tuned primary 

 oscillatory circuit. The coupling was always extremely 

 loose. Trains of damped oscillations, of maximum amplitude 

 of the order of a few hundredths of an ampere could be 

 excited in this primary ; and hence feeble electrical oscilla- 

 tions of calculable magnitude could be excited in the 

 secondary. The amplitude of the oscillatory electromotive 

 force at the coherer contact, and the energy delivered to the 

 coherer, can be computed from the amplitude of the primary 

 oscillations ; but, besides other considerations, the lack of 

 facilities for measuring the small mutual inductance between 

 primary and secondary made another method (described below) 

 of determining the energy seem preferable. 



The trains of damped oscillations in the primary were 

 produced by making and breaking a small current running- 

 through the primary inductance. In consequence, the 

 coherer yielded a sound in the telephone connected in series. 

 As a general rule, the telephone sound is greater the greater 

 the amplitude of the oscillations in the primary, because the 

 response of the coherer is greater. To measure the response 

 of the coherer, the intensity of the sound was balanced 

 against another sound which was produced in the same 

 telephone by the interruption of an independent source of 

 direct current, which could be switched into the circuit 

 of telephone and interruptor in the place of the coherer. 

 In this way the behaviour of the detector could be quan- 

 titatively recorded when the steady current through it, 

 or the oscillatory energy delivered to it, underwent desired 

 variations. The method has enabled me to attain some 

 degree of consistency in measuring the effects of oscillatory 

 voltages as low as 4 \ 7 volt, and energy transformations of 

 the order of a few thousandths of an erg. 



Only one type of coherer has been examined closely. This 

 is a mercury and oxidized iron coherer — a kind I have used 



