88 Messrs. Ayrton, Mather, and Sumpner 



" set-up ammeter " producing an image on the same scale as 

 is used for the electromagnetic d'Arsonval galvanometer. 

 With a i{ set-up ammeter " the suspended system is supported 

 by means of an almost torsionless phosphor-bronze strip, 

 requiring many twists to be given to it to bring the spot of 

 light on to the scale when the current, that is to be kept 

 constant, is flowing through it. A motion, then, of the 

 spot of light over 100 divisions of the scale corresponds 

 perhaps to a variation of only 20 in this current ; and con- 

 sequently, by means of such an instrument and a suitable 

 adjustable resistance, a current can be kept constant to a very 

 small fraction per cent. 



It might be objected to our conclusions that, in the papers 

 read by Prof. Threlfall before this Society, and published in 

 the ' Philosophical Magazine ' for December 1889, he comes 

 to the conclusion that coils used in what we may shortly call the 

 Mudford way (figs. 5, 6, and 7) give greater sensibility than 

 when used in the Gray way (figs. 8-12). But on studying 

 Prof. Threlfall's papers we find that, from various numerical 

 errors, he makes his coils when used as a Gray galvanometer 

 one ninth as sensitive as they really were, and when used 

 as a Mudford galvanometer seventeen times as sensitive as they 

 were ; so that Prof. Threlfall's ratio of the sensibility of the 

 Gray galvanometer to the sensibility of the same coils when 

 used as a Mudford galvanometer requires to be multiplied by 

 the very large number 153*. 



In order to correct Prof. Thelfall's calculations, the following 

 alterations must be made : — 



Phil. Mag., Dec. 1889 : Page 465, line 11, for " The cur- 

 rent was therefore 1'26 x 10 -7 amperes/' read " The current 

 was therefore 1*43 x 10~ 8 amperes." 



Same page, line 25, for " 2*5 x 10~ 8 amperes " read 

 " 2-8 x lO" 9 amperes." 



Page 469, line 1: " The galvanometer having been brought 

 to a state of sensitiveness of 5 scale-divisions for 10~ n amperes, 

 the measurement of the resistance of the sample of sulphur 

 in question became a tolerably easy matter." On examining, 

 however, the results obtained in the actual measurements 

 with the sulphur (p. 471), we find that the sensibility of the 

 galvanometer was not as above stated, 500,000 divisions per 

 micro-ampere, but 34,200. And, as his scale was 3 metres 

 away from the galvanometer, and his scale-divisions 1 millim., 

 this number 34,200 reduced to our standard becomes 22,800 f. 



* These errors were also noticed by Prof. A. Gray, and referred 

 to in an article in the Phil. Mag. for February 1890. 



t Prof. Threlfall says, in a letter published in the Phil. Mag. for June 

 1890, in reply to these criticisms, that his galvanometer was not in a state 



