324 Prof. J. A. Fleming and Mr. J. E. Petavel : 



minating beam, whilst the light falling on the left-hand side 

 of the photometer disk is a uniformly illuminating beam of 

 the same quality and colour coming from the same part of the 

 arc but not interrupted, and representing therefore the mean 

 value of the light emitted from that selected area of the arc. 

 Observers were deputed to measure all the various quan- 

 tities by the different instruments, and a power of constant 

 definite amount was supplied to the electric arc from an 

 alternating-current machine driven by a continuous-current 

 motor. By means of the carbon resistance and the reflecting 

 wattmeter this power was kept constant at a selected value 

 for a sufficient time to enable all the various periodic quan- 

 tities to be observed at sufficiently frequent intervals during 

 the phase. In all cases the arc was allowed to burn quietly 

 for half an hour to get the carbons into a constant position 

 before any observations were taken. It is hardly necessary 

 to go into the details of delineating the current and electro- 

 motive-force curves, as the process of doing this is now well 

 understood. The Kelvin vertical multicellular voltmeter, 

 having a half microfarad condenser placed across its ter- 

 minals, was employed for the measurement of the potential 

 difference of the carbons in the following manner : — 



The voltmeter, with its associated condenser, was connected, 

 through the contact-maker driven by the shaft of the alter- 

 nating motor, to the carbons of the arc ; the contact-maker 

 thus closed the circuit at a certain instant during the phase, 

 and the voltmeter gave the instantaneous value of the potential- 

 difference of the carbons. In series with the arc was placed 

 a non-inductive resistance of suitable magnitude. A switch 

 was arranged so that the voltmeter with the condenser in 

 parallel with it, both being in series with the revolving 

 contact-maker, could be put across either the terminals of 

 this non-inductive resistance, or else between the carbons of 

 the arc. By rocking over the arm carrying the spring- 

 brushes of the contact-breaker, the voltmeter circuit was 

 closed at a particular instant during the phase, and the volt- 

 meter reading gave therefore, when corrected, the instanta- 

 neous value either of the potential-difference of the carbons 

 or the instantaneous value of the current through the arc. 

 As the Kelvin multicellular voltmeter used by us only begins 

 to read at 60 volts, in order to get readings for lower values 

 than 60 volts, it is necessary to add a known electromotive 

 force to the voltmeter circuit in order to block up the needle 

 of the voltmeter to a false zero. This was done by connecting 

 a known number of small Lithanode secondary batteries, the 



