MAGNETIC EFFECT OF ELECTRIC CURRENT. 1 1 I 



the force action between the coils is measured, first for a current 

 /' and then for a current /", the ratio I' \I" is equal to the 

 square root of the ratio of the observed force actions. The elec- 

 trodynamometer of Siemens is used for measuring current ratios 

 in this way. A general view of this instrument is shown in Fig. 

 760. The movable coil B (see Fig. 76^) is suspended by a fine 



Fig. 76a. 



Fig. 76b. 



thread and its terminals dip into two mercury cups which permit 

 of its being connected in series with the fixed coil A. When 

 current is allowed to flow through the two coils in series, a torque 

 is exerted upon the movable coil by the fixed coil, and the helical 

 spring is twisted, by turning the milled head, until the movable 

 coil is brought to its standard position, as indicated by the 

 pointer which is attached to B. The angle through which the 

 milled head is turned is indicated by the pointer which is at- 

 tached to the milled head, and this angle is a measure of the 

 force action between the coils so that this angle is proportional 

 to the square of the current, or the current is proportional to the 

 square root of the angle. 



