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 LXIY. Intelligence and Miscellaneous Articles, 



SILK V. WIRE, OR THE " GHOST " IN THE GALVANOMETER. 

 BY R. H. M. BOSANQUET. 



^HE ballistic galvanometer which I used for some years for the 

 -*- measurement of induction currents consisted of a small astatic 

 pair of needles with mirror, surrounded by a small coil of very low 

 resistance. The suspension was from a silk fibre about six inches 

 long, the fibre being left just stout enough to carry the weight. 

 This whole combination was extremely sensitive and for the most 

 part convenient to work with. 



The silk suspension, however, has certain troublesome properties. 

 I shall not here enter into the manner in which it gradually 

 untwists itself when stretched, or into its property of taking a set 

 from any change of position ; but shall confine myself to the 

 appearance which we called the "ghost." 



At certain times the needles of the galvanometerwould move about 

 with sudden and capricious movements, the mirror often traversing 

 several degrees of the scale. The decision and sharpness of the 

 movements were very remarkable, and we habitually spoke of their 

 cause as the " ghost." 



The ghost used to visit us mostly in summer between the hours 

 of nine and eleven in the forenoon, and about six in the evening *. 

 "When these movements began it was no use attempting to work 

 with the galvanometer. There can be no doubt that the movements 

 were due to the solar heat falling more or less directly on the 

 instrument and causing hygroscopic changes in the silk fibre. 



In the early summer of this year I found it necessary to free 

 myself from this source of interruption, and constructed a galvano- 

 meter with a wire suspension. The difficulty consists in combining 

 a needle system large enough to vibrate very slowly on the wire 

 suspension, with a coil having sufficient power over the needles, 

 and at the same time a low enough resistance. 



The needles are stout knitting-needles seven inches long. They 

 are hung from a support fastened to the wall by a very fine wire 

 about 5 feet long. The needles are very nearly astatic, and the 

 complete double vibration takes a little over half a minute. The 

 coil consists of about 500 turns of JNo. 20 B. W. G. The resistance 

 is much greater than that of the old instrument and the loss of 

 sensitiveness is an inconvenience ; but the instrument works well 

 in connection with our large earth induction-coil of 250 turns of 

 the same wire, and it is entirely free from the visits of the ghost. 



It is my conviction that silk and thread suspensions are sources 

 of error and inconvenience to an extent that has been imperfectly 

 realized; and that they ought to be entirely banished from all 

 instruments of precision. 



* The aspect of the galvanometer-room is north and east. 



