TRANSACTIONS OF SECTION A. 571 



From initial concjitiops — 



TLnabf 



^0 = l^IT^H) 



and hence the deflection 



a UnabQ /n^ 



"max. ^ . - (••••• \^J 



From equations (1) and (2) it is possible to design an ordinary galvanometer ; 

 the only other consideration is the damping. 



Now the retarding couple on the coil due to damping 



= {llnabf^ (RinC.G.S.) 



Introducing this term into the equation of motion, 



K 

 and critical damping takes place when 



rv=l^"«^ C.G.S. units . . . . (3) 



In order that the motion of the coil may be eifectively stopped by short- 

 circuiting, its resistance must not be greater than the value given by this equation. 

 When a deflection is being read, however, in order to render the damping term 

 inappreciable the total resistance of the circuit must be considerably greater than 

 this value. This can be readily arranged by suitable connections. 



From these equations it is seen that the only method by which it is possible to 

 decrease the period and still keep the sensibility constant is by decreasing the 

 control C, If the period is brought down by other means there will be a loss of 

 sensibility. 



In a special case a galvanometer was required to have a very low period with 

 a fairly high sensibility and quick working, and this investigation was undertaken 

 in order to obtain a satisfactory instrument. 



From calculations founded on the above equations a design was made, 

 assuming a certain size of coil and shape of magnet, which was sent to a certain 

 firm of instrument-makers, so that they could inform us of the value of the strength 

 of the magnetic field in the gap. This was found to be 230 lines per square centi- 

 metre, and from experiments on the torsion of phosphor bronze strip it was found 

 that a control of '01 would not be diflicult to obtain. 



Assuming a coil, length 5 cm., breadth 1'72 cm., a control of -01 (dyne cm.), 

 a periodic time of 80 sees., and a sensibility of § radiam per microcoulomb (i.e. 

 ^ radiam as motion of coil), then the coil must contain 22 turns of No. 28 wire. 



On testing for damping the critical resistance is found to be 7"1 ohms, which, 

 being much larger than the resistance of the coil, indicates that the motion of the 

 ooil could be easily stopped by short-circuiting. 



From these results the galvanometer was constructed, and was found to have a 

 much higher period and much lower sensibility than it had been designed for. 

 The cause of this was not far to seek, lying in the fact that the copper wire used 

 in the construction of the coil was magnetic, and it was found that this magnetic 

 control was very large in comparison with the feeble control of the strip. 



To overcome this difiiculty various devices were tried, and wire was obtained 

 from several manufacturers, in the hope that it would prove non-magnetic. 

 The best results were obtained, however, by depositing copper electrolytically on 

 a very fine wire. The resulting copper wire was fairly non-magnetic, but so 

 brittle that it could not be used. To improve the wire it was rolled between 

 brass rollers and annealed ; but in doing so some very fine iron dust floating in the 

 air of the workshop must have become deposited on it, for it was again found to 



