130 



DR. E. H. GRIFFITHS AND MR. EZER GRIFFITHS ON THE 



position of the galvanometer spot when the battery was disconnected was that due 

 to any thermo-electric currents existing in the bridge and its connections, and thus 

 any movement viable on establishing the battery circuit was attributable to that 

 circuit only. The key, however, presented some novel features. Brass segments were 



Pt-Ag 



Fig. 3. 



fixed on a vertical spindle in such a manner that when the pointer was at 0, the 

 galvanometer circuit alone was complete, when rotated through 120 both galvano- 

 meter and battery circuit were established, and on a further rotation through 120 

 the battery was reversed. The segments were so devised that induced currents 

 during the " makes " and " breaks " would not affect the galvanometer. The whole 

 series of operations could thus be performed very rapidly by one turn of the spindle. 



The galvanometer was one of the original Paschen type. Its four coils were wound 

 with wire whose diameters increased with their distance from the centre. 



Four coils, each of about 5 ohms and separately adjustable, were used. It was 

 desirable for the present work to obtain a system whose period of oscillation would be 

 small, which would rapidly settle to its final position and yet have great sensitiveness. 



For such a purpose it is advisable to use a system whose moment of inertia is 

 reduced as far as possible. The type, constructed by ourselves, consisted of two 

 groups, each containing 18 magnets astatically arranged. The extreme length of 

 the longer magnets was about 1 ^ mm. The whole system, together with the mirror 

 and the connecting glass fibre, weighed less than 11 mgf. It was suspended by a 

 quartz thread about 17 cm. long and between 3 and 4 /x diameter. The clearance 

 allowed by the ovals in the coils was but a fraction of a millimetre and the faces of 

 the coils were almost in contact, these faces being coated with tinfoil, to promote 

 the damping of the oscillations by electro-magnetic induction. Reckoned on the 

 usual scale, the sensitiveness of this galvanometer could have been easily raised 

 beyond 10" 1 ", but by exterior magnetic control we reduced it until by one reversal of 

 battery a deflection of 1 mm. indicated about 1B[ j, 00 Pt as we found that, owing to 

 the wandering of the zero point, a higher degree of sensitiveness detracted from, 

 rather than increased, the accuracy of our observations. 



The galvanometer had to be placed at a considerable distance (about 13 m.) 



