44 REPORT—1883. 
On the under side of the ebonite, strong strips of copper, with their 
faces well amalgamated, are screwed, forming with the holes in the 
ebonite a series of cups, which are filled with mercury. 
The copper strips are cut, as shown in the figure, to make the 
necessary connexions. The distances between the holes is such that the 
electrodes of the three coils respectively fit into ab, cd, and ef, or into 
a; bi, cd’, and.e’ f'. 
Connexion is made with the bridge by means of the cups A, B, while 
the electrodes of the second single unit coil fit into g and h. In the first 
position the three coils are in multiple arc, as will be seen from the 
figure, and can be compared with a single unit, while in the second they 
are in series with the other single unit, and can be compared with the 
10 units. 
By this contrivance the 10 unit is referred to the single standard. 
To determine the value of a coil of 100 units, the three 3 units can be 
replaced by three 30 units, and the single units by tens. 
This, however, is not the most convenient method for the total re- 
sistance if the wire of the Fleming bridge in use is only =\, of a unit, thus 
affording too small a range for the ready comparison of large resistances. 
The following has been adopted :—Four coils are arranged as in a 
Wheatstone’s Bridge, one being the 100 units to be tested, two of the 
others in opposite arms, two known 10 units, and the fourth a known 
single unit. 
These coils are all arranged in the same circular trough of water and 
their electrodes dip into four mercury cups. 
Tf all the coils are correct no current will traverse the galvanometer. 
Of course in practice this condition is never realised. Hither one of the 
ten units or the single unit is too great. Let us suppose it is the latter ; 
connect its two electrodes with the two electrodes of a resistance box and 
take out plugs from this till a balance is secured. Then if the resistance 
of the ten units be Q and R, that of the single unit 8, and the shunt W, 
the resistance of the shunted arm is sate and that of the 100 
7 
Beg eG Ma D 
WS 
Now, in practice, if Q, R, S are fairly accurate, W will be a large 
resistance, and an approximate knowledge of W will suffice. W may 
thus, for all we require, be taken from a resistance box by a good maker 
which has stood for some time in the room in which the experiments are 
conducted, the temperature being taken as that of the room. A box 
has been ordered from Messrs. Elliott Brothers, to be used for this and 
similar purposes. 
The same firm have also supplied a high resistance galvanometer for 
the testing. 
Of course if one of the ten unit coils is too great, then the shunt W 
must be put in with it. 
In accordance with the resolution of the Committee, a fee of Il. 1s. 
has been charged for testing single units, and of 1. 11s. 6d. for others. 
The only coils the testing of which is regularly undertaken are single 
units and multiples of single units by some powers of 10. 
But though this is so, two standard ohms have been ordered, using 
for the value of the B. A. unit -9867 ohms., and when they arrive and have 
been tested, it will be easy to determine the value of coils which do not 
