874 Dr. W. H. Eccles on Coherers. 



expression for x, the current through the detector at break of 

 primary current, is of the form 



x — le~ at + he~ mt cos (pt + q)+ke- bt cos (ct + d), 



where m and p belong to the primary circuit and 6 and c to 

 the secondary circuit, the coupling being very feeble. The 

 coefficients /, h, k are complicated functions of the electrical 

 dimensions of the circuits. Hence the energy delivered to 

 the detector at a break appears as six terms, of which, with 

 the dimensions adopted, only two are of numerical importance, 

 namely, 



h 2 l±m 3 2lh/m. 



The other terms are less than 1 per cent, of the larger of these. 

 Both h and I invoke m, the damping factor of the primary, 

 but not prominently. This fact enabled m to be determined 

 easily and with sufficient accuracy. A calibrated thermo- 

 galvanometer with a heater and series resistance totalling 

 1000 ohms was put in the usual place of the detector ; the 

 mutual inductance between the circuits was increased (but 

 the coupling was still very small}, the speed of the interruptor 

 was raised, and heavier primary currents were used than was 

 customary when a detector was in position. Thus a con- 

 siderable deflexion of the thermogalvanometer was obtained 

 and recorded. Then a short loop of very thin copper wire of 

 known high-frequency resistance was put in series with L', 

 and the observation repeated. Since the energy received by 

 the thermogalvanometer is proportional inversely to the 

 resistance of the primary on each occasion, the high-frequency 

 resistance of the coil L' can be calculated from the obser- 

 vations. The mean of several determinations gave 0'78ohm. 

 The steady-current resistance was 0*22 ohm. 



The power delivered to the detector in the standard con- 

 ditions of the circuits and primary currents can be computed 

 as just indicated. For accurate results it is necessary to know 

 M, the mutual inductance between primary and secondary, 

 and 7/2, the primary damping factor, very accurately. As 

 neither of these could be determined with the precision 

 desirable, the thermogalvanometer was invoked to determine 

 the power passing from primary to secondary. In fact, the 

 experiment to determine m, just described, may be taken as 

 determining the factor of transference of power. Similar 

 observations to the above were therefore made with primary 

 currents of different values and with rather high speeds of 

 interruption (nearly 200 per second), and the fact was 

 established, obvious theoretically, that the power passed 

 to the thermogalvanometer was proportional to the square 

 of the primary current and to the frequency of interruption. 



