510 Prof. K. Threlfall on Sensitive Galvanometers. 



state here that I have gone back to Mr. Boys's method of pre- 

 paring the quartz, which (as might have been expected) is much 

 better than mine. I wonder how many people have any real 

 idea of what a sensitiveness of " five divisions to 10~ n ampere " 

 really means. From the glibness with which we talk of sensi- 

 tiveness, I fancy the conception has not generally been really 

 grasped. Let me give an illustration. We all look on the 

 eye as perhaps our most sensitive organ. In the Philosophical 

 Magazine for January 1889 is a paper by Professor Langley^ 

 in which an estimate is made of the minimum rate of expen- 

 diture of energy (by radiation) which can affect the eye. The 

 estimates are of course rough and not very concordant, and 

 there is some obscurity arising from the hoary-headed diffi- 

 culty of distinguishing between work and rate of working. 

 However, it follows from the data given, that an expenditure 

 of work at the rate of 2'6 x 10~ 6 C.G.S. per second will affect 

 the eye so as to produce vision when the light has a wave- 

 length nearly that of the line A; while an expenditure at the 

 rate of 2*6 xlO" 11 (or 5*7 x 10~ 12 from another part of the 

 paper) will produce the same effect if the light is green. 

 Now the galvanometer I described had a resistance of nearly 

 16,000 ohms; and since 10 -11 ampere produced a deflexion of 

 five divisions, we may say that we could detect a current of 

 10~ 12 ampere (giving a deflexion of half a division). In this 

 case a simple calculation (without arithmetical error) shows 

 that the galvanometer absorbed work at a rate of about 

 1*6 xlO -9 ergs per second. Consequently the sensitiveness 

 of the galvanometer may be said to stand half way between 

 that of the eye for crimson and green light respectively. 



I never intended to institute a comparison between Prof. 

 Gray's form of galvanometer and the Thomson form : what I 

 wanted was to get a workable instrument as easily as possible, 

 and I found the Thomson form most easily manageable. 

 Though the Gray form may be workable at the hands of such 

 exceptionally skilful manipulators as its inventors, I do not 

 think that for the ordinary run of experimentalists it is to be 

 compared with the older form, and I certainly do not pin any 

 faith to silk fibres a few inches long. Though, as Professor 

 Gray remarks, his form of galvanometer possesses " solid 

 advantages/' as it certainly does, still it possesses also solid 

 disadvantages, and these will, 1 think, in the long run be 

 found to predominate. 



I am, Gentlemen, 



Yours obediently, 



University of Sydney, N.S.W., RlCHARD THRELFALL. 



March 7, 1890. 





