and a new Secondary Battery. 



53 



(d) A Thomson's astatic reflecting galvanometer, whose in- 

 ternal resistance was rather less than *6 B.A. unit. The deflec- 

 tions were read off in the usual manner, on a scale which was so 

 graduated that 300 divisions occupied a space of 19 centims. 

 The image of the slit appeared as a band of light 4 millims. 

 broad, the position of one edge of which, sharply defined, gave 

 the index-reading. By using a lens sufficiently large for the 

 scale to be viewed by both eyes at once, the error of observation 

 did not exceed *5 of one division, or, in the deflections generally 

 obtained, "3 per cent, of each reading. Attached to the galva- 

 nometer was a shunt, by which 9 per cent, or 99 per cent, of the 

 current could be diverted at pleasure from the galvanometer-coils. 

 The directing magnetic bar was placed at the top of its support, 

 and with the same orientation as the upper needle. 



The index-readings taken were in all cases those correspond- 

 ing to the extreme swing of the galvanometer-needle. 



(e) A set of resistance-coils from 1 up to 13,000 B.A. units. 

 (/) A key, by means of which the current from the Daniell 



cell could from time to time be sent to the galvanometer through 

 the same resistance-coils as the secondary current. 



(g) Other apparatus, mentioned in the account of its em- 

 ployment. 



3. The connexions are shown in the accompanying figure 

 (fig. 1), where A is the galvanometer, B the shunt, C the com- 



mutator, D the Daniell cell (of which P and N are the poles), 

 M the Morse key (of which d is the anvil on which the spring 

 presses the lever), Bthe resistance-coils, S the secondary couple 

 (of which a and b are the plates), and Z the key (§ 2/). When 

 the commutator is so turned that a is connected with P, N is in 

 metallic communication with b through the lever of the Morse 

 key; and the secondary couple is being excited ; but on depress- 

 ing the lever, b is shunted from N to the galvanometer, and the 

 secondary circuit is completed through the resistance-coils to a. 



