284 LECTURES TO SCIENCE TEACHERS. 



That in this room coincides pretty nearly with the length 

 of the room. The larger instrument is, therefore, im 

 properly placed, being across the room, so that it would 

 have to be turned round. The circle being properly set, 

 you know also that the strength of the current is in pro- 

 portion, not to the deflection of the magnet, but to the 

 tangent of that deflection; so that if the magnet is 

 deflected through any angle, the strength of the current 

 is proportional to the tangent of this angle. Instruments 

 such as these are therefore usually termed Tangent Gal- 

 vanometers ' } galvanometer being the name given to any 

 instrument which measures the strength of a current by 

 the electro-magnetic action of that current. 



Here is an instrument, one of the first, I believe, of 

 the kind which was constructed ; it would not however, 

 easily give us an absolute measurement. There is a magnet 

 and a coil of wire surrounding it, so that when a current 

 passes through the coil the magnet is deflected ; but the 

 section of the coil which is wound on this elongated reel 

 is not circular, so that it would involve a very complicated 

 calculation in order to ascertain the mean distance of the 

 various parts of the current from the magnet. Such an 

 instrument could hardly be made to serve for absolute 

 measurement, but it will serve very well for comparative 

 measurements, and for that purpose it is so arranged that 

 the coil can turn independently of the magnet, and it is 

 used in this way : If we send a current through the coil 

 and that deflects the magnet, then the coil and the plate 

 which carries it are turned round until a fixed mark is 

 brought to coincide with an index attached to the magnet, 

 and the measurement depends on the extent to which this 

 circle is turned. It is an instrument which will give very 

 accurate comparative measurements, but not absolute 

 measurements. On the table there are various forms of 

 instruments for giving comparative measurements, and one 

 also for giving absolute measurements. There is a 

 magnet suspended inside this copper-box, and it is in 

 the form of a magnetized steel ring. The thick mass of 

 copper which surrounds it is for the purpose of making 

 it come rapidly to rest when displaced. Probably you 

 know that when a magnet swings in the neighbourhood of 

 a large mass of copper or other conducting material, its 



