50 Mr. T. Gray on Silk v. Wire Suspensions in 



to about # 001 , and m in such an instrument as we are consi- 

 dering will not differ much from unity. Hence r must be 

 much less than *001; and we find, from the tables given above, 

 that, for a fibre of about the usual length, say 5 centim., t will 

 be about '0003; or about one fourth of the total force is, in this 

 case, due to the fibre. This, then, may be taken as about the 

 limit of sensibility beyond which we cannot easily pass with an 

 ordinary Thomson's astatic galvanometer with small needles ; 

 to get beyond it, attention must be directed to an increase of 

 m*. The limit here indicated is, however, far beyond anything 

 that can be reached with wire suspension, the smallest current 

 which can be measured being about 10 ~ 9 ampere for a galva- 

 nometer of 1 ohm resistance, and about 0"2 x 10~ 10 ampere for 

 one of 10,000 ohms resistance. When I— -F is as much as 

 0*01, or between a tenth and a twentieth of the earth's hori- 

 zontal force in this country, the effect of the set of the sus- 

 pending fibre is extremely small. With such a value of I— F, 

 however, a properly constructed galvanometer, the resistance 

 of which is as low as 1 ohm, will measure a current of 10~ 8 

 ampere. When very high sensibility is absolutely necessary, 

 it may be to some extent obtained by increasing the length of 

 the fibre ; but if this prove insufficient, some alteration or 

 other arrangement of the parts becomes necessary. Such an 

 arrangement is described in the paper referred to in the foot- 

 note ; but it may be remarked that, in so far as this arrange- 

 ment is intended to increase m, it is only important when 

 I — V is made practically zero. So long as I — I' is consider- 

 ably greater than r, a high value of m is of no importance ; 

 and the Thomson form is, because of the small inertia of its 

 needle system, decidedly the best. 



Note. — Since the above was written Mr. J. T. Bottomley 

 has suggested to me that some interesting results might be 

 obtained if the vibrational method, above referred to, were 

 carried out with the fibre and vibrator in a good Sprengel 

 vacuum ; and in conjunction with him I have made some 

 preliminary experiments, the results of which seem worth 

 quoting. 



The vibrator used was the lighter of the two referred to in 

 connection with the former experiments, and consisted of a 

 small ring of brass 0*295 centimetre radius and 0'012 gramme 

 in weight. It was suspended, as shown in the diagram, in- 

 side a small spherical bottle provided with a long neck and a 

 ground stopper, to the lower end of w T hich the fibre was 

 attached. A tube passed from the side of the bottle to one 



* On this subject see a paper u On a New Reflecting G-alvanometer of 

 Great Sensibility," by T. and A. Gray, Proc. Roy. Soc. No. 230 (1884). 



