MAGNETIC EFFECT OF ELECTRIC CURRENT. 



109 



and, since this relation is true for every element of the circular coil, it follows that 

 the total torque is equal to IH times the total area A enclosed by the turn of wire, 



that is 



T=AIH (40) 



in which T is in dyne-centimeters, / is in abamperes, and A is in square centi- 

 meters, H being expressed in gausses. If the coil has more than one turn of wire, A is 

 equal to the sum of the areas of all the turns. Thus, if the coil has four turns of wire 

 of which the radii are a, b, c and d respectively then A = ira 2 -f- icb 2 -f- ire 2 + ird 2 . 



Fig. 74. 



59. The electrodynamometer is an instrument for determining 

 strength of an electric current from a measurement of the mutual 

 force action between two coils of wire through both of which the 

 current flows. One of these coils is fixed and the other is 

 suspended. The magnetic field produced by the fixed coil exerts 

 a force upon the suspended coil, and this force, or the movement 

 which it produces, is observed. When the coils are very simple 

 in shape it is possible to calculate (from geometrical and me- 

 chanical data alone) the force action between the two coils for 

 a given current, or, conversely, to calculate the value of the cur- 

 rent when the force action is observed in mechanical units. 

 Such an electrodynamometer is called an absolute electrodyna- 

 mometer. 



