

Sensitive Galvanometers. 209 



resistance taken out of an ordinary bridge-box. The ter- 

 minals of the bridge-box were coupled up through the gal- 

 vanometer to a megohm. ... In the final test the period of 

 vibration of the magnet system was 80 seconds, and the re- 

 sistance out of the bridge-box was 100 ohms. The E.M.F. 

 acting through the megohm and galvanometer and 100 ohms 



was therefore tttt™ Clark cells, say # 0145 volt. The current 

 10100 J 



was therefore z. kf^tk = 1'26 x 10~ 7 amperes/' Lower down 



Prof. Threlfall says that this gave a double deflexion of 5 

 divisions, and that therefore the current per division was 

 " 2-5 x lO" 8 amperes." 



Now this calculation, if I understand the arrangement 

 (and taking the statement that *0145 volt worked " through 

 a megohm, the galvanometer, and 100 ohms," there seems 

 no room for mistake), is evidently erroneous. The current for 



•0145 

 the 5 divisions' double deflexion was really 101fin ^ ampere, or 



1*43 x 10 -8 ampere nearly, and the current per division 

 2'85 x 10 -9 ampere, or 5'7 x 10~ 9 ampere per division of single 

 deflexion. Thus Prof. Threlfall, by an error in arithmetic, 

 makes the sensibility of his instrument only about -J- of what 

 it was in reality, according to the data which he gives. 



For this sensibility it is to be noted the period of vibration 

 of the needle system was 80 seconds, the suspension (two 

 silk fibres) 16 centimetres long, and the distance from the 

 galvanometer-mirror to the scale 155 centimetres. 



Prof. Threlfall then describes an instrument of another form 

 which he constructed with the coils of the unsuccessful instru- 

 ment of our form, and begins his next paper with the follow- 

 ing statement of its sensibility : — " The galvanometer having 

 been brought to a state of sensitiveness of 5 scale-divisions 

 for 10 -11 amperes, the measurement of the resistance of the 

 sample of sulphur in question became a tolerably easy matter." 

 One naturally infers from this statement that the sensibility 

 of the galvanometer in the experiments thereafter described 

 was 2 x 10~ 12 ampere per division of deflexion. However, 

 calculating from the table on p. 472, we find that it was 

 really about 3x 10 -11 ampere per division of single deflexion. 

 If a double deflexion is meant in the statement above, this is 

 only about -J- of the sensibility stated, or, if a single deflexion 

 is meant, ^ of the sensibility stated. That the mstromeni 

 was working near the limit of its sensibility seems evident 



