548 REPORT— 1902. 



the iiiti'i'vening space being occupied by trees and hilly ground, so that the 

 receiver was screened by this ground from the oscillator. 



When a telephone was placed iu the receiving circuit the separate dots and 

 dashes sent from the oscillator were distinctly heard without any failure or inter- 

 mission. 



The addition of a fishing-rod 12 feet long, through the rings of which was 

 passed a stout iron wire, to the top of the receiving copper surface, so as to increase 

 the virtual height of this surface, made a marked improvement iu loudness, the 

 various dots and dashes being now heard without putting the telephone to the ear. 

 The decoherence was perfectly good for the telephone, but when the telephone 

 was replaced bj a relay there was not sufficient automatic decoherence ; a slight 

 tap was generally necessary. 



I have always used telephones as the indicators of oscillations, and employed 

 three or four coherers in parallel in the telephonic circuit. 



With the exception of the coherer just described, I have never hit upon one 

 which automatically and satisfactorily decohered, and I do not know if any coherer 

 hitherto discovered decoheres automatically with a relay. 



The most severe test of any coherer occurs when the coherer is employed close 

 to the oscillator — that is, in the laboratory — and it appeai-s to me that the above 

 coherer stands this test remarkably well when used with a telephone. 



8, A Graphical Method of Determining the Discharge Curve of a Condenser 

 through a variable Self-induction. By E. W. Marchant, D.Sc. 



The method employed is an extension of that described by Dr, Sumpner in 

 a paper to the ' Phil. Mag.' in 1897 for determining the rise in the cun-ent through 

 a coil of variable self-induction. The method consists in graphically determining 



successive values of ,, the rate of change of current for the discharge. 



where «'= current in amperes. 

 _ ._ fidt Eo = capacity of condenser in farads. 



di " oil R R = resistance of circuit in ohms. 



^^ = -r ■ ,T L = self-induction of circuit in henrys. 



:-- + ^ -y . Eq = initial P.D; to which the condenser 



•'*' "^ ^'-^ was charged. 



The method was first tested for the case in which the self-induction is con- 

 stant, the time interval between successive determinations of the value of * being 



dt " 



so chosen that ^^^ is a simple number K, so that the quantity -'-^ = Ki, for the 

 Clt ' 01\ 



time di and can be represented as a fraction of i. Plotting in this way it was found 



that the periodic time for an oscillatory discharge was obtained accurately to within 



1 per cent., and that the maximum value of the current was determined accurately 



to within 2 per cent. For a discharge through a circuit of larger resistance it was 



found that the period of oscillation was again accurate to within 1 per cent., and 



that the damping ert'ect, due to the resistance and the displacement of the time 



of maximum current from an instant midway between two zero values, was well 



shown. 



The method was then applied to the determination of the discharge curve 



through a coil having a soft iron core, and with which a number of experiments 



had been made. Three curves were shown for the same circuit, in which the 



voltages applied to the condenser were respectively 9,400 (2-5 mm. spark) 2,350, 



and 450. In the first two cases the current wave after clinging to the zero line 



becomes approximately sinusoidal, while in the third the curve is sharply peaked. 



The periodic times given by these determinations were shown to be in approximate 



agreement with those found experimentally. The phenomena due to hysteresis in 



the iron core were also shown in the third curve. 



