374 



SUBMARINE CABLE LAYING AND EEPAIKING 



work. Contact between the coil and instrument terminals 

 is made by means of two platinum points PS, PS, on the 

 under side of the frame, which press on a pair of springs in the 

 base of the instrument and make contacb when the frame is 

 in position. The coil is of fine copper wire wound on an 

 aluminium frame (Fig. 223) and suspended top and bottom by 

 thin strips of phosphor bronze, which connect the ends of the 

 coil electrically with the termiaals of the instrument. There 

 is also a fixed cylindrical soft-iron core inside the coil, so that 



Fig. 221. — Latest Pattern Sullivan Shipboard Galvanometer. 



the coil is in a strong magnetic field. The tension on the 

 suspension is adjusted at the lower end by a set screw and at 

 the top a torsion head is provided by which the coil can be 

 turned round the axis of suspension for bringing the spot of 

 light to zero. A mirror is fixed to the top of the coil at its 

 centre, by which the deflections are observed in the usual way 

 by a reflected ray of light. A window is provided in the brass 

 cover through which the rays of light pass, and an opening at 

 the top for access to the torsion head, by means of which the 

 spot can be adjusted to any required zero. 



