240 THE ELECTRIC TELEGRAPH. 



the ring a wooden frame is supported, which keeps it in form, 

 and carries a compass-box, containing the magnetic-needle. 

 The latter is short and is cemented to the middle of a long 

 glass fibre, which serves as a pointer, and allows the divisions 

 of the scale over which it moves to be of a considerable size. " 

 The ring, with its continuations, is supported upon a tripod 

 with levelling screws, in which it is turnable for facility of 

 placing it in the plane of the magnetic meridian. The 

 distance of the ring from the needle renders the latter per- 

 fectly insensible to weak currents, and a multiplier becomes 

 necessary. 



Messrs. Siemens have constructed a tangent galvanometer 

 in which the copper ring is replaced by four separate, thick, 

 well-insulated copper wires, bent in form of a circle of about 

 the same diameter as the ring in Pouillet's instrument, and 

 terminating round the pedestal on which they are supported 

 in four pairs of brass terminal screws. With this arrange- 

 ment the galvanometer can be made twice, thrice, or four 

 times as sensitive by letting the current pass as many times 

 round the needle. It may also be used as a differential gal- 

 vanometer by letting the current pass in reverse directions 

 through the convolutions. 



The magnetic needle with its pointer of glass or aluminium 

 is suspended at the end of a fibre of unspun silk hung from 

 an adjusting screw, on the top of a glass tube, and is lowered 

 on to the card by turning the screw when the instrument is 

 not in use. The support turns upon a vertical axis, by which 

 the coil may be placed north and south. 



Gagain constructed a galvanometer in which he professes 

 to have succeeded in reducing the error arising from the 

 altered position of the magnet, by removing the plane of the 

 ring to a distance of half its radius from the centre of the 

 needle, by which, when the latter is deflected, the one half 

 is just so much more as the other half is less strongly acted 

 upon. Bravais, who undertook the mathematical demonstra- 

 tion of the correctness of Gagain's theory, has proved that 

 when a magnetic needle is subjected to the action of a 

 circular current in the magnetic meridian, the centre of the 





