162 3 REPORT—1862. 
unless special precaution were taken against it, as pointed out by Professor 
W. Thomson*. To avoid this source of error, the galvanometer circuit is 
broken between 3 and 4, figs. 7 and 8, at K,, and should only be closed after 
the battery circuit has been completed at K and equilibrium established 
throughout all the conductors. 
Before passing to a detailed description of the apparatus as actually con- 
structed, some remarks are required as to the means of making temporary 
connexions. All connexions which require to be altered may be the means 
of introducing errors, inasmuch as the points of contact are very apt to offer 
a sensible but uncertain resistance. In measuring small resistances, the 
resistance at the common binding-screws is found to create very considerable 
errors. Binding-screws have therefore to be ayoided at all points where an 
uncertain resistance could cause error. Mercury-cups, made as follows, have 
been found in practice very suitable for temporary connexions, and have been 
adopted in the apparatus. The bottom of each cup is a stout copper plate, 
with its surface well amalgamated, forming one of the two terminals to be 
joined. A stout copper wire, 1 inch in diameter, with a flat end well amal- 
gamated, forms the other terminal. When the amalgamation is good, and 
care is taken that the wire shall rest on the plate, this form of connexion 
offers no sensible resistance. The amalgamated wire is easily kept bright 
and clean by being dipped from time to time in a solution of chloride of 
mercury and wiped. The copper plate should also be removed from the cup, 
cleaned, and re-amalgamated occasionally, All permanent connexions should 
be soldered. 
The apparatus itself, as actually constructed, will now be described (figs. 1 
to 6). It consists of a wooden board, about 12 in. x 7 in., containing the 
mercury-cups, the adjusting wire, WX, the key, K, and the terminals to which 
the battery and galvanometer are connected. The letters in the figures 
1 to 6 correspond exactly to those used in the diagrams 7 and 8; and the 
apparent complexity of the connexions can thus be easily disentangled. 
cc,, aa, are two pairs of mercury-cups, into which the terminal wires on the 
bobbin, C, A, dip. This bobbin contains the two coils, C and A, forming 
the arms of the balance. rr, and ss, are mercury-cups, into which the 
terminals of the standard and coil to be adjusted are placed. These mercury- 
cups are so connected with the four cups, d,d,, f,f,, that when d is con- 
nected with d,, and f with f,, by a couple of wires in a small square of wood, 
D, then A, C, 8, and R are connected as in fig. ’7; but when D is turned round, 
so as to connect d with f, and d, with f,, A, C, R, and S are connected as in 
fig. 8. D is called the commutator. The same end might be effected without 
a commutator by simply interchanging R and §; but it is frequently incon- 
venient to do this. All these connexions are made by short stout copper bars, 
dotted in fig. 2. The wire WX, the sliding brass piece H, carrying a spring 
for the contact at U (fig. 4), and the scale E, by which the position of H is 
observed, will be readily understood from the drawing. The sliding piece, H, 
is connected with the proper points by the helix of copper wire, A, and the 
serew, I. GG, and BB, are common binding-screws, to which the wires 
from the galyanometer and battery are attached. K is the key, by depress- 
ing which, first, the battery is thrown into circuit, and then the galvano- 
meter. It consists of three brass springs, 1, 2, 3 (fig. 5), each insulated 
one from the other, and connected by three screws, 1, 2, 3 (fig. 2), with the 
necessary points of the arrangement. A fourth terminal, 4 (figs. 2 and 6), 
* Vide Phil. Mag, August 1862. 
