1871. ]} Proceedings of the Asiatic Society. 221 
To measure the Resistance of a Non-Electromotor. 
Insert this resistance in the battery branch, and repeat the pre- 
ceding observations obtaining respectively deflections a,° and 6,° 
then if 7 be the resistance to be found 
200 X tan 3,° 
| ae tan a,° — tan b,° 
Eliminating R from this and the preceding equation we have 
Be tan 6° ) U 
tan a,° — tan b,° tan a° — tan 0° \ 
— 1 Siemens’ Units. 
r= 200 ( 
The most constant electromotive force that can be practically 
made use of is that of anew Menotti’s cell with clean zine and 
copper, a standard cell as it is called. The electromotive force of 
such a cell is therefore taken as our unit of electromotive force. 
To express the Electromotive Force of a Battery in terms of that of 
Standard cell. 
Join the two poles of the battery to the two binding screws 
respectively of the galvanometer, and plug up holes marked B and 
2000: then the battery and the thin coil only are in circuit. Let the 
deflection be 7°. 
Remove the plug from hole marked 2000, but leave plug in 
B: then the battery, the thin coil, and 2000 S. U. are in circuit. 
Let the deflection be m°. 
Repeat these two tests with the standard cell substituted for the 
battery. Let the two deflections now obtained be p° and q°. 
Then if E be the electromotive force of the battery, and e that 
of the standard cell, 
tan 1° & tan m° _ tan p° — tan g° 
~ tan p° X tan g° ~*~ tan 2 — tan m° qe 
This galvanometer can also be used as a telegraphic receiving 
instrument but then the two copper stops, 8, 8’; shown in the dia- 
gram, on pl. iv, should be placed one on each side of the alumin- 
ium needle. If well adjusted, signals can be read which are 
produced by ten Menotti’s cells at the other end of a line about 
400 miles long consisting of No. 5} wire and havng an insulation 
of two millions per mile. 
