equal electrical resistances. 343 
or, by (1) and (2), 
dag iet] EW An a easel lense: 
ata(-+.)-ptaleta) cee ecceecces (6). 
From this equation the difference between 1/A and 1/B is found, 
when the shunts a,, a, 0,, b, are known. 
We also have, from (6), 
A Sa eel es 
AB =p- 475 (a, Tin) Oa. Del 
PARTS oe ais melitrz rl 
pence Asi 2 ia . Gi De ») 
When A and B are very nearly equal, it will be sufficient to use 
A? or B® instead of AB on the right side, or merely to use the 
nominal values of A and B on that side. 
§ 3. Practical details. The measurements are easily made 
when suitable connecting pieces are used. In laboratories where 
serious comparisons of resistance are made, mercury cups formed 
in massive blocks of copper would be used in making the con- 
nexions. But mercury cups are out of place in a crowded practical 
class where the students have only a limited time in which to do 
the experiments ; from the demonstrator’s point of view the most 
important thing is that there should be nothing which can “go 
wrong.” 
Fig. 3. 
In Fig. 3, Z, M, N, O, P, Q represent six strips of stout copper 
with forked ends for clamping under the terminals of the coils. 
The pieces L, O are connected by about 15 cm. of stout copper 
wire and the pieces NV, Q are connected in a similar manner*. 
Short wires are soldered to L, M, N, P, as shown in Fig. 3, and by 
* Copper wire is preferable to any stiffer connexion, since its use allows Z and 
N to be moved without straining the terminals of the coils C and D. 
