210 On Sensitive Galvanometers. 



from the fact stated by the author that the zero was " always 

 on the move." 



It is to be noted that in this instrument the suspension was 

 a quartz fibre 85 centim. long, and that the distance of the 

 scale from the mirror appears (p. 466) to have been 3 metres. 

 (Hence for a scale at a distance of 1 metre from the mirror 

 the current for 1 division of single deflexion would be 9 x 10 ~ n 

 ampere.) 



Thus in his comparison of the two instruments Prof. Threl- 

 fall evidently states the sensibility of his own instrument as 

 either 15 or 7*5 times its real amount, and certainly makes 

 the sensibility of the trial instrument of our form only about 

 -^ of what it was according to his own observations. 



In my own experiments the arrangement was as follows : — 

 A circuit was made of a DanielFs cell (freshly prepared) and 

 two resistances, one of 22,000 ohms and the other of 100 ohms. 

 The terminals of the galvanometer were applied at those of 

 the resistance of 100 ohms. The galvanometer had a free 

 period of 23 seconds, and the double deflexion produced was 

 88 scale-divisions each ^q of an inch. Hence the current was 

 nearly 1*5 x 10 -7 ampere. For one half millimetre single 

 deflexion the current therefore was 2*7 x 10~ 9 ampere. 



To compare this with Prof. ThrelfaD/s result with the same 

 type of instrument, we ought to reduce to a scale 1*5 

 metre from the mirror. The current for 1 division would 

 then be 1'85 x 10~ 9 ampere as against 5*7 x 10~ 9 for Prof. 

 Threlfairs ; or, with only 23 seconds period, my instrument 

 has 3 times the sensibility his had with a period of 80 seconds. 

 With the latter period (and that with even a longer period, the 

 instrument can be readily used, I am certain from our former 

 experiments) the current would be about -^ of that stated, or 

 1*5 x 10~ 10 ampere per division ; that is, the instrument would 

 have about 36 times the sensibility of that made by Prof. 

 Threlfall. The silk fibre was a little less than 3 inches long, 

 and was by no means so fine as it might be made. 



A previous set of experiments, after which the fibre was 

 broken down, and the instrument dismounted for resuspension, 

 gave greater sensibility. The electromotive force of the cell 

 used was, however, in that case a little uncertain. 



I ought to state that the sensibility of the instrument 

 depends very much on its state of adjustment. As the time 

 at my disposal for these experiments was very limited, and I 

 am not able to apply anything like the degree of skill which 

 my brother used in the management of the instrument, and 

 in the arrangement of a delicate silk-fibre suspension, the 

 sensibility is not now so great as that which we formerly 



