VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
225 
experiments. R is the resistance of a coil of platinum-silver wire made at Professor 
Stuart’s workshops, according to the pattern designed by Professor Fleming, of 
Nottingham, 
The wire was supplied by Messrs. Elliott Brothers, and is that used by them in 
the manufacture of coils of about 100 units resistance. Pt, as has been stated, is about 
160 units. 
N6w suppose that the connexions P P', Q Q' are broken, and that P Q is connected 
by means of one of the p-j pieces. Break M N and connect L K, then the battery 
circuit is complete. Now connect M N again, the secondary circuit is complete, and 
the current running in the primary. Reverse the commutator C, an induction current 
is produced in the secondary circuit, and may be measured by the throw of the 
galvanometer needle. 
To measure the primary current M N is broken and M L and H H' are connected. 
Between H and L the primary current is divided; the resistance in the direct cir¬ 
cuit H Y L being about 1 ohm, that in the circuit H S G Q PM L about 3072 ohms, 
so that about 3 0 ] 7 3 of the whole current runs through the galvanometer; the perma¬ 
nent deflection of the galvanometer G is observed, and from this the value of the 
current is calculated. 
We turn now to a detailed description of the apparatus used. 
The coils were wound by Professor Chrystal. 
Two brass rings were carefully turned and a rectangular channel cut in the outer 
limb of each. A slit was cut in each ring to prevent currents in the frame (this was 
of course unnecessary for our experiments, but might render the coils more useful in 
many cases); the slit was closed with a piece of insulating material into which 
binding-screws connected with the wires of the coil are screwed. 
The coils are lettered A and B. 
In the experiments the coils are to be placed with their planes parallel and their 
centres on a line at right angles to their planes. 
A cylindrical brass rod was carefully turned and fixed to a brass support so that it 
would stand in a vertical position. 
Two tripod pieces of brass of the form shown in fig. 2 were constructed. The brass 
rod fits accurately through the aperture O in the centre of the tripod, and the plane 
of the arms 0 A, 0 B, O C is at right angles to the axis of the rod. The curved 
surfaces A A l5 B B x , C O l5 are small portions of the same circular cylinder whose axis 
coincides with that of the rod, and the radius of this cylinder is the same as that of 
the carefully turned inner surface of the annulus on which the coils were wound. 
This annulus thus would then fit on to A A ]} B B 1} C C x and rest on the flat 
surfaces A A : A 3 A 3 , B B 1 B., B 3 , C C x C 2 , C 3 , which are all parts of the same 
plane at right angles to the axis of the bar. If then we place one coil, A suppose, in 
this manner on the tripod, its plane is perpendicular to the axis of the bar, and its 
centre lies on that axis. 
mdccclxxxiii. 2 G 
