i66 



ELEMENTARY LESSONS ON [CHAP. in. 



to equal increments of the tangent. If the scale were 

 divided thus, the readings would be proportional to 

 the tangents. It is, however, harder to divide an arc 



Fig. 90. 



into tangent-lines with accuracy than to divide it into 

 equal degrees ; hence this graduation, though convenient, 

 is not used where great accuracy is needed. 



2OO. Absolute Measure of Current by Tangent 

 G-alvanometer. The strength of a current may be 

 determined in " absolute " units by the aid of the 

 tangent galvanometer if the " constants " of the instru- 

 ment are known. The tangent of the angle of deflection 

 represents (see Art. 124) the ratio between the magnetic 

 force due to the current and the horizontal component of 

 the earth's magnetic force. Both these forces act on the 

 needle, and depend upon the degree of its magnetisa- 

 tion ; but as they both depend equally upon its magne- 

 tisation, the ratio between them is independent of the 

 magnetic moment of the needle, which, therefore, we 

 need not know for this purpose. We know that the 

 force exerted by the current at centre of the coil is pro- 

 portional to the horizontal force of the earth's magnetism 

 multiplied by the tangent of the angle of deflection. 

 These two quantities can be found from the tables, and 

 from them we calculate the absolute value of the current, 

 as follows : Let r represent the radius of the galvano- 

 meter coil (measured in centimetres) ; its total length (if 

 of one turn only) is 2<7?r. The distance from the centre' 



