52 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
been kept under observation in the Metrology Department. Possibly there is a flaw 
in the rod, but it is certain its length has not kept so constant as other rods which 
were not baked and immersed in wax. We decided, therefore, to omit the treatment 
in the case of the four cylinders for the Lorenz apparatus. 
The cylinders were turned in a manner very similar to that employed for the 
suspended coils of the current balance. The inner and end surfaces of a cylinder 
were turned with the cylinder fixed to the face plate of the lathe, but the outer 
surfaces and double spiral grooves were cut when the cylinder was mounted on a 
specially constructed mandrel supported between dead centres. The inner and outer 
surfaces of a cylinder are practically concentric and the ends at right angles to the 
axis. 
The turning of each cylinder occupied about four weeks. The cylinders referred 
to as Nos. 1 and 2 were turned by the late Mr. Taylerson of the Engineering 
Department of the Laboratory. Mr. Taylerson had marked ability for accurate 
work of this kind and made many useful suggestions during the progress of the work. 
The turning of cylinders Nos. 3 and 4 was very ably done by Mr. Tribe of the 
Engineering Department. 
The winding of the coils was carried out in a manner identical with that employed 
for the coils of the current balance. It is only necessary to state here that the coils 
were wound with the wire in tension, the effective load on the wire during winding 
being 4 kgr. The cylinders were rotated very slowly and after each revolution a 
stoppage was made for measurements to be taken of the diameter of the wire. 
The leads of all the coils must lie in a plane containing the axis of the cylinder, for 
otherwise the mutual inductance of the leads and the discs will not be zero. To 
ensure the absence of any correction due to such 
a cause, the connections to the coils were made in 
the manner shown in fig. 8. At the points where 
the coils terminate, two radial cylindrical holes 
a and b are drilled and enlarged to admit of the 
slotted brass nipples NN' being screwed into 
them. Soldered connections are made between 
the nipples NN', the leads c and d, and the leads 
through a and b. The leads c and d pass to two 
brass blocks mounted on an ebonite piece screwed 
to one end of the cylinder, and to these same 
blocks a small concentric cable is secured, in the manner illustrated in the figure. The 
whole of the leads shown in the figure lie in an axial plane of the coil and this same 
plane contains the leads connected to the other coil on the cylinder. 
An estimate was made of the accuracy with which the number of turns is known. 
On each cylinder there are two coils each having 96 turns and a diameter of about 
36 cm.; the length of wire to each coil is, therefore, about 108 m. From observations 
