256 Electricity and Galvanism. 



first instance made with the battery. Like the former current, this 

 is only of momentary duration. From this experiment we learn, that, 

 when a current traverses a wire, it induces or excites another current 

 in any conductor held parallel to it, a second being excited the 

 instant the first current ceases to traverse the wire. These currents 

 are respectively named primary and secondary, and are always op- 

 posed in direction, the primary current moving in an opposite di- 

 rection to the battery or exciting current. If, instead of using a 

 battery current as an exciting agent, I had plunged a magnet into 

 the centre of the coil connected with the galvanometer, the electric 

 equilibrium of the wire would in like manner be disturbed, a primary 

 current being induced on first introducing the magnet, and a secon- 

 dary one on withdrawing it. It is obvious that if, by any contri- 

 vance, contact with the battery could in the first example be rapidly 

 made and broken, as, in the second, the magnet be as quickly im- 

 mersed and withdrawn, we should procure a rapid series of currents 

 moving alternately in opposite directions; and on this is founded 

 the construction of all the magneto-electric and electro-magnetic 

 machines. 



Numerous forms of electro-magnetic machines have been suggested 

 for medical purposes, and it is really not a matter of any importance 

 which you employ, provided care be taken to have the one you have 

 chosen so arranged as to allow of a sufficiently copious development 

 of electricity. As we have seen that in all such contrivances a small 

 voltaic current furnishes the initial force, it is important to have this 

 completely under command, and to be able to make and break con- 

 tact with the inducing apparatus, with the utmost facility and rapidity. 

 You may break contact with the battery, if you please, by means of 

 a ratchet or cog-wheel ; but this is often inconvenient, as it renders the 

 services of an assistant necessary. On this account an automatic 

 apparatus is always to be preferred. I believe I proposed the first 

 of these several years ago in the Annals of Philosophy ; but this, as 

 well as all others I have seen, are much inferior to one constructed 

 by an ingenious philosophical instrument-maker, Mr. Neeves, of 

 Broad Street, Holborn, and this is the only one I ever now employ. 

 It possesses the advantage of simplicity, facility of employment, 



