SCIENCE AND PRACTICE. 243 



This galvanometer is shown in perspective in Fig. 125. 

 The bed of the instrument supporting the coils, c c, is turn- 

 able about a centre in the tripod levelling stand, d d\ its 

 circumference is divided into degrees of arc. 



The silk fibre by which the needle system is suspended 

 is attached to an adjusting screw in the middle of the cross 

 beam b b, resting upon the upright pillars, a a. A glass 

 cylindrical case and glass top protect the instrument from 

 dust and the fibre and needle from currents of air. The 

 lower needle swings in the centre of the coils of wire ; the 

 upper one acts as a pointer to and is suspended over a 

 graduated card; its position being observed through the 

 telescope, e. 



The multiplier is wound on two bobbins, which are placed 

 side by side, with the needle system between them. They 

 are of about the same length and magnetic action, and may 

 be used either separately or together. 



These instruments were used for testing the Malta- Alex- 

 andria cable and others, whose electrical conditions have 

 been under the surveillance of Messrs. Siemens. In their 

 instruments, each of the bobbins have about 3,500 units' 

 resistance and about 12,000 turns, making a total resist- 

 ance of 7,000 units, and, in all, 24,000 turns round the 

 needle. The card inside is graduated from the line, 0, 

 parallel to the direction of the coils, to 90 on each 

 side. 



These galvanometers may be made of almost any required 

 sensitiveness for weak currents, by making the needle system 

 sufficiently astatic. 



18. The astatic condition of a pair of needles is measured 

 by the time which it occupies in making an oscillation across 

 the magnetic meridian. Matteucci had a pair which took 

 seventy seconds to make a single oscillation ; but from five 

 to ten seconds is a very convenient degree of directive force 

 to obtain for the measurement of high resistances by weak 

 currents, otherwise the needle system is liable to change its 

 zero by trifling disturbances over which the operator has 

 no control. 



B 2 



