280 LECTURES TO SCIENCE TEACHERS. 



measurement. Again, an electric current passed through 

 a compound liquid almost always causes chemical 

 change a decomposition of the substance. The general 

 name given to processes of this kind is electrolysis the 

 electrical breaking up of the substance. And the electro- 

 lytic action of currents might be employed to furnish a 

 third method of measuring their strength. 



In order to employ any one of these principles for the 

 measurement of currents, the conditions under which these 

 actions take place must be rendered as simple and definite 

 as possible. Without going into minute particulars I may 

 remind you to take the electro magnetic action first that 

 the force exerted by an electric current upon a magnet 

 depends partly on the strength of the current, partly upon 

 the relative positions of the magnet and of the conductor 

 in which the current passes. This I may almost say is self- 

 evident, that a change of relative position would change 

 the effect. One kind of change of relative position would 

 be a change of distance : the farther a current is from a 

 magnet, obviously the less it will act upon it ; but inde- 

 pendently of changes of distance, a change of direction 

 will in general cause a change of action ; so that, to express 

 the matter in general terms, we may say that the effect of 

 a current upon a magnet depends on the relative positions 

 of the two. It also depends upon the length of the con- 

 ductor which is in the neighbourhood of the magnet. That, 

 in a certain sense, is merely a repetition of what I said as 

 to position, and will be evident at once. If you take, for 

 instance, what I have here a magnet suspended on a pivot 

 at the centre of a graduated circle, with a circular con- 

 ductor formed by a coil of wire which would enable us 

 to pass a current many times round the magnet, and 

 terminated by wires by which it can be connected with 

 external conductors if those wires are carried far away 

 they may go to such a distance that the distant portion 

 of them does not exert any sensible influence on the magnet. 

 So that we have to consider the length of the conductor 

 near the magnet, not the total length of the conducting 

 wire. We might have a current sent through this coil 

 from a mile or 100 miles away, but the portion of the con- 

 ductor at that distance would obviously exert no sensible 

 effect. So then, the force exerted upon the magnet depends 



