Units in the Electromagnetic Unit. 433 



K.f\Q 



to a distance of .,- ,. ,. /w , centim., we get Y—Y / = -904187 



(c.g.s.) io > ouo ; * 7 



The E.M.F. of Thomson's gravity Daniell was measured by 

 comparing it before and after the above experiment directly 

 with that of the above battery by means of Sir William Thom- 

 son's quadrant electrometer. The E.M.F., e, of the cell was 



e= JrJl =0-034380 C.G.S. electrostatic units, 

 zb'zyy 



(B) Absolute Electromagnetic Measurement of the E.M.F. 



This measurement was made by determining the strength 

 of the current given by the E.M.F. by means of a tangent- 

 galvanometer, and then measuring the resistance of the circuit 

 in the way to be described presently. 



The tangent-galvanometer employed consists of a circular 

 coil, of mean radius 18*2 centime., containing 400 turns, in 

 19 layers, of insulated copper wire, the breadth and the depth 

 of the coil being 2 and 1*3 centims. respectively. The needle 

 of the galvanometer consists of a magnet only about -| centim. 

 long, made of hard-tempered steel wire, and suspended in the 

 centre of the coil by a single silk fibre. To the needle is 

 attached a very fine straight glass fibre, of such a length that 

 its ends travel round a graduated dial of radius a little less 

 than that of the coil, thus serving for taking readings. 



The mathematical theory shows that, in a tangent-galvano- 

 meter, 



Hx/^TFtan a 3gV; 



2^n ' Sq 2 rlxd\^-iy ' ' W 



where c is the current-strength, H the horizontal component of 

 earth-magnetism, « the angle of deflection, n the number of 

 turns of wire in the coil, r the mean radius of the coil, b half 

 the breadth of the coil in the plane at right angles to the 

 plane of the coil, d half the depth of the coil in its plane, and 

 q the number of layers in the coil. If E be the E.M.F. pro- 

 ducing the current c in a circuit of resistance K, then, by 

 Ohm's law and from the preceding equation, we get 



2m ' 3gVj + tf , (0»—l)" ' { ) 



The formula (2) shows that, in order to measure an E.M.F. 

 in absolute electromagnetic units, we have to determine (a) the 

 deflection a, (b) the resistance R, and (c) the horizontal com- 

 ponent of earth-magnetism H. 



