274 Mr. W. H. Preece. On a New Standard of 



adjusted at will ; the strength of the latter is read on a graduated 

 electrodynamometer, or upon a galvanometer. 



For laboratory purposes a more accurate mode of measuring the 

 intensity of the current is the following :* — 



G is a galvanometer wound with two coils, one (5) consisting of a 

 short length of thick wire, and the other (r) of a long length of thin 

 wire. In circuit with the latter coil is a resistance, p, and a standard 

 battery E. 



The relative deflective values of the coils s and r for similar currents 

 being as 1 to n, then when balance is obtained on the galvanometer, 

 the current C flowing through the coil s will be — 



r 



r being the resistance of the galvanometer coil and battery, and E the 

 E.M.F. of the battery E. 



Thus if the relative deflective values of the coils s and r are as 1 to 

 473, that is, if w=473, then if E consist, say, of a 5-cell Daniell 

 battery, and the resistance (r) of the galvanometer coil and battery 

 be 800 ohms, the current C, when balance is obtained by adjustment 

 of p, will be — 



c _ 473x5xl-07 _ 2530 

 800+_p 800+_p* 



Practically with this arrangement a change of 1 * th of an 

 ampere in the current through the coil 5 can easily be detected, and 

 currents can be read to three places of decimals. 



To work the instrument it is only necessary to place the screen 

 (D) in the place whose illumination is to be determined, and then 

 by means of the rheostat (R) to lower or increase the current passing 

 through the lamp until the grease spot, when viewed through the 

 eyepiece, becomes invisible, then the light reflected on the diaphragm 

 and transmitted through the grease spot by the distant surface of 

 drawing paper will be equal to the light thrown upon the opposite 

 side of the diaphragm by the glow lamp. The current producing 

 the light thus becomes a measure of the illumination of the 

 screen. 



The following measurements were made to determine the current 

 that gave various degrees of illumination — the source of light being 

 a standard candle. 



* Cardew, " Proceedings of Telegraph-Engineers and Electricians," vol. ii, 

 p. 301. 



