168 ELEMENTARY LESSONS ON [CHAP. in. 



enough angle, will overtake the needle, which will once 

 more lie parallel to the coil. In this position two forces 

 are acting on the needle : the directive force of the 

 earth's magnetism acting along the magnetic meridian, 

 and the force due to the current passing in the coil, 

 which tends to thrust the poles of the needle out at 

 right angles ; in fact there is a " couple " which exactly 

 balances the " couple " due to terrestrial magnetism. 

 Now it was shown in the Lesson on the Laws of Mag- 

 netic Force (Art. 123), that when a needle is deflected 

 the " moment " of the couple is proportional to the sine 

 of the angle of deflection. Hence in the sine galvano- 

 meter, when the coil has been turned round so that the 

 needle once more lies along it, the strength of the current 

 in the coil is proportional to the sine of the angle through 

 which the coil has been turned. 1 



2O2. The Mirror Galvanometer. When a gal- 

 vanometer of great delicacy is needed, the moving parts 

 must be made very light and small. To watch the 

 movements of a very small needle an index of some 

 kind must be used ; indeed, in the tangent galvanometer 

 it is usual to fasten to the short stout needle a delicate 

 stiff pointer of aluminium. A far better method is to 

 fasten to the needle a very light mirror of silvered glass, 

 by means of which a beam of light can be reflected on 

 to a scale, so that every slightest motion of the needle 

 is magnified and made apparent. The mirror galvano- 



l Again the student who desires to compare the strength of two currents 

 will require the help of a Table of natural sines, like that given on page in. 

 Suppose that with current C the coils had to be turned through an angle of 

 degrees ; and that with a different current C' the coils had to be turned 

 through ff degrees, then 



C : C = sin : sin ff. 



It is of course assumed that the instrument is provided with a scale of 

 degrees on which to read off the angle through which the coils have been 

 turned. It is possible here also, for rough purposes, to graduate the circle 

 not in degrees of arc but in portions corresponding to equal additional 

 values of the sine. The student should try this way of dividing a circle 

 ->fter reading the note On Ways of Reckoning Angles, p. IOQ. 



