ELECTRICAL APPARATUS. 165 



In order to make an electric current follow a prescribed path 

 between two conductors which are kept at different potentials, 

 this path must not only be occupied by conducting matter, but 

 this must be insulated by non-conducting matter from any con- 

 ductors through which the current is not intended to pass. In 

 most cases copper wires form the most convenient channels for 

 conveying a current in a required direction, and the best insula- 

 tion is obtained by surrounding them with a sufficiently thick 

 stratum of air ; but when there is not room for a sufficient thick- 

 ness of air, as when wires have to be coiled closely together, or 

 when air is altogether excluded, as in the case of submarine tele- 

 graph wires, its place may be supplied by a covering of cotton, 

 woollen, silk, gutta-percha, india-rubber, or some other solid non- 

 conductor. 



The apparatus employed for detecting the existence of electric 

 currents and indicating their direction are termed galvanoscopes ; 

 instruments which serve also for measuring the strength of cur- 

 rents are called galvanometers. The general principle on which 

 all galvanoscopes and most galvanometers hitherto constructed 

 depend, may be stated as follows : 



A magnet and a conducting circuit traversed by an electric current 

 tend to assume such relative positions that the magnetic force acting 

 across the area bounded by the circuit is a maximum. In applying 

 this principle it must be borne in mind that a conducting circuit 

 traversed by a current whose apparent direction agrees with the 

 motion of the hands of a watch is equivalent to a magnet whose 

 south pole is towards the eye. The condition stated above is 

 fulfilled when the magnetic forces due to the current and to the 

 magnet act in the same direction, and when the whole force of 

 the magnet acts across the area enclosed by the current. The 

 fact that a mutual force is exerted between a stationary conductor 

 traversed by a current and a magnet was discovered by Oersted in 

 1820, and very soon afterwards (1821) the ordinary form of galvano- 

 meter, with a multiplying coil and pair of astatic needles, was 

 constructed almost simultaneously by Schweigger and by Poggen- 



