230 MEASUREMENT OF CURRENTS. 



The coil consists of two cylindrical parts, which are almost in 

 contact, each comprising in its core half the copper sphere. When 

 an astatic system is used, two similar needles are thus placed, each 

 in the centre of the coil, in which the current circulates in contrary 

 directions. 



In order to modify the external field, two equal magnets, having 

 their centres in the axis of rotation, are placed under the base of the 

 instrument, and by means of a suitable screw-gear may be placed in 

 any given direction, either in reference to each other, or in respect of 

 the instrument. The time of oscillation is much greater than in the 

 apparatus on Thomson's plan. 



851. VARIOUS INSTRUMENTS. In connection with the instru- 

 ments we have described, and which are the principal types at 

 present used in scientific researches, we shall mention a few others, 

 which in some of their features have a theoretical interest. 



We will describe, first of all, the marine galvanometer of Sir 

 W. Thomson. The motion of a vessel, and its change of direction, 

 make it impossible to utilise the terrestrial field. The coil is placed 

 between the limbs of a horse-shoe magnet, which produces a sensibly 

 uniform magnetic field, and such that the axis of the needle, in its 

 position of equilibrium, is almost in the line of the poles. The field 

 of the earth is so weak compared with that of the magnet, that its 

 action is scarcely perceptible. It is almost completely suppressed, as 

 well as that of the masses of iron in the vessel, if we enclose the 

 entire instrument in a thick iron cylinder, which forms a magnetic 

 screen. A correcting magnet, parallel to the needle, and having its 

 poles opposite those of the fixed magnet, may be displaced parallel 

 to itself by means of a screw. The pendulum motions of the needle 

 are got rid of by suspending it from a short silk fibre, attached to a 

 spring, which exerts a suitable strain. The resistance of these galva- 

 nometers may be as much as 30,000 or 40,000 ohms. 



852. In order to give galvanometers considerable directive force 

 and great fixity of zero, M. Deprez* also uses the powerful field of a 

 horse-shoe magnet, but he replaces the ordinary magnetised needle 

 by a system of soft iron needles, mounted parallel to each other on 

 the same axis (fish-back needle). As the magnetisation of the needle 

 is almost proportional to the intensity of the resultant field, the 

 directing couple and the action of the current are very powerful, 

 and the time of oscillations is very small. On the other hand, the 

 electromotive force of induction produced by the motion of the 



* DEPREZ. Journal de Physique, Vol. ix., p. 227. 1880. 



