150 Silk v. Wire in Galvanometers. 



If a small weight with a pointer is suspended from a silk 

 fibre, and it is brought to rest and left covered under glass, 

 the position will continue slowly to change for some days. 

 This is what I regard as a consequence of structural twist. It 

 and the taking of set combined lead to the phenomenon of the 

 change of zero in galvanometers, leaving the " ghost " out of 

 consideration for the present. 



With me, sensitiveness is far from being the first require- 

 ment in galvanometers. The elimination of elements of an 

 inconstant character stands first. Now, when we so greatly 

 diminish the moment of the needles as to admit of the pro- 

 perties of the silk suspension causing changes of the zero, which 

 is admittedly the case, we have a demonstration that changes 

 depending on the silk are able to influence the results. The 

 increase in the length of the silk, according to my experience, 

 increases some at least of the irregular effects instead of dimi- 

 nishing them. By the elimination of the silk we can obtain 

 instruments quite sufficiently sensitive for practical purposes, 

 and entirely free from errors which I regard as introduced by 

 an incorrect identification of refinement with accuracy. 



Since my former note was written I have wound a pair of 

 coils for the galvanometer in question, consisting of about 

 500 turns of 16 B.W.G. By employing a cement of shellac 

 varnish thickened with red lead I was able to wind these solid 

 without any frame, and so to get all the thick wire into the 

 same space as before. I thus obtain nearly the same delicacy 

 as with the old silk suspension. The clearness of the indica- 

 tions, as compared with those of the old galvanometer, is 

 evidenced by the facility with which the thermoelectric cur- 

 rents, arising from the binding- screws of the circuit, are 

 identified, a considerable deflection being produced by laying 

 the finger on the brass binding-screw. With the silk such 

 effects were not so easily isolated, movements often continuing 

 even though the circuit was not joined up. 



Mr. Gray deprecates going back, " something like half a 

 century/' to galvanometers with large needles. I doubt 

 whether the older experimenters realized the advantages of 

 wire suspension. But I have always wondered that so little 

 weight is now attached to the deliberate opinion of Gauss, 

 that accuracy in such measurements is to be best attained by 

 enlarging the dimensions of the apparatus. In this matter I 

 am occasionally tempted to think that the old is better. 



