﻿486 Mr. F. L. 0. Wadsworth on a New Method of 



The first galvanometer experimented upon was a fine 

 Thomson instrument, whose coils were wound by While, and 

 whose needle system was made by Very*. The magnetic 

 system was built up of hollow cylindrical magnets, rolled up 

 out of the thinnest sheet-steel, heated red-hot in fused ferro- 

 cyanide of potassium, and hardened in mercury in a powerful 

 magnetic field. The system was then astaticized by the usual 

 method of stroking with a weak bar-magnet. Every care 

 was taken to secure the maximum intensity of magnetization. 

 The constant of the galvanometer in its most sensitive state 

 was C=l-5x 10-9 1- 



The best value of the constant of the Allegheny galvano- 

 meter already referred to was = 1*3 x 10 ~ 9 . On October 23 

 the magnetic system was removed and remagnetized, and 

 reastaticized by the method described above. No other 

 change was made, The new constant after magnetization 

 was C = 8x 10" 10 , an increase in delicacy of nearly 100 per 

 cent. 



The second galvanometer to whose magnetic system this 

 method of treatment was applied was one constructed by 

 Queen and Co. especially for the Observatory. The mag- 

 netic system had very nearly the same dimensions as the one 

 already described, but the individual magnets were solid. 

 The method of hardening and magnetizing was not described 

 by the makers. The constant for this galvanometer as re- 

 ceived from them was : — 



C = l'6x 10 -9 for coils in series, R=309 ohms, 



C = 5'5 x 10 -9 for coils in parallel, R=19*2 ohms. 



The needle was removed and remagnetized by the new 

 method and the constant redetermined. The new constant 

 was: — 



C = lx 10 -10 for coils in series, 



C = 3*4x 10 -11 for coils in parallel. 



Part of the immense improvement here is due to a more 

 accurate centering of the coils, to the magnetic system, and 

 to a reduction of the excessive damping ; yet with a most 



* This instrument was a duplicate of the celebrated Thomson galvano- 

 meter, with Very hollow magnets, used by Langley at Allegheny in his 

 bolometric work ; and which was considered at that time to be the most 

 sensitive (for its resistance) in existence. See paper by S. P. Langley, 

 " On hitherto unrecognized Wave-lengths," Phil. Mag. [5] xxii. p. 149 

 (1886). 



f C = current in amperes which produces a deflexion of 1 millim. on 

 a scale at the distance of 1 metre, when the time of a single swing is 

 ten seconds. 



