CHAP, in.] ELECTRICITY AND MAGNETISM. 167 



to all parts of the coil is of course r. From our defini- 

 tion of the unit of strength of current (Art. 196), it 



follows that 1 i x 2 ~ = force (in dynes) at centre, 



or i x ** = H tan d ; 



r 



hence - i H tan d. 



27T 



The quantity --- is called the " constant " of the galva- 

 nometer. Hence we obtain the value of the current in 

 absolute (electromagnetic) units 1 by multiplying together 

 the galvanometer constant, the horizontal magnetic force 

 at the place, and the tangent of the angle of deflection. 

 Tangent galvanometers are often made with more than 



one turn of wire. In this case the " constant " is 



2TTH 



where n is the number of turns in the coil. 



2O1. Sine Galvanometer. The disadvantage of 

 the tangent galvanometer just described is that it is not 

 very sensitive, because the coil is necessarily very large 

 as compared with the needle, and therefore far away 

 from it. A galvanometer with a smaller coil or a larger 

 needle could not be used as a tangent galvanometer, 

 though it would be more sensitive. Any sensitive 

 galvanometer in which the needle is directed by the 

 earth's magnetism can, however, be used as a Sine 

 Galvanometer, provided the frame on which the coils 

 are wound is capable of being turned round a central 

 axis. When the instrument is so constructed, the 

 following method of measuring currents is adopted. 

 The coils are first set parallel to the needle (i.e. in the 

 magnetic meridian) ; the current is then sent through 

 it, producing a deflection ; the coil itself is rotated round 

 in the same sense, and, if turned round through a wide 



1 The student will leani (Art. 196 and 323) that the practical unit of 

 current which we call "one weberper second" is only ^ of one "absolute" 

 unit of the centimetre-gramme-second system. 



