RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
53 
on the radial holes and a consideration of the methods adopted for drilling them, the 
number of turns is considered to be correct within 2 parts in 1,000,000. 
The copper wire with which the eight coils are wound was supplied by The London 
Electric Wire Company on bobbins of the same diameter as the cylinders. It is hard 
drawn No. 24 S.W.G. and its mean diameter, obtained from about 800 measurements 
taken when winding the coils, is 0'557 5 mm. The diameter was also measured in the 
machine employed to determine the diameter of the coils, the mean of eighty measure¬ 
ments being identical with that already given. 
Measurement of the Mean Diameters of the Coils. 
Twelve axial planes at angular distances of 15 degrees apart are marked on the end 
faces and on the ungrooved portions of the outer cylindrical surfaces of each cylinder. 
These planes are numbered 1 to 12 and are the reference planes in the diametral 
measurements. Each cylinder was mounted in turn on the mandrel employed for 
turning the outer surfaces and supported between dead centres attached to the 
measuring machine. 
This machine was made by Sir W. G. Armstrong, Whitworth & Co. at their Openshaw 
Works, to designs prepared by the firm in collaboration with Mr. L. F. Richardson, 
formerly of the Metrology Division of the Laboratory. It consists of a straight bed 
carrying two fixed headstocks with cone-centres, between which the coil, on its 
mandrel, can be mounted. A saddle sliding on the bed carries two measuring head- 
stocks on a slide which is adjustable so as to bring the line of centres of these head- 
stocks exactly perpendicular to that of the fixed headstocks. Each of the measuring 
headstocks contains a barrel which can be moved in and out along the line of centres 
by means of a carefully calibrated micrometer screw ; and sliding freely in the centre 
of each barrel is a plunger, the front end of which constitutes the measuring face, 
while the rear end carries a small knife-edge pressing against a vertical lever 
pivoted in the barrel. At its upper end this lever carries a sensitive spirit 
level. The barrel is advanced by means of the micrometer screw until the 
plunger, being arrested by contact with the object to be measured, tilts the level 
far enough to bring the bubble to a definite mark. The reading is then taken 
with the aid of a vernier on the measuring wheel attached to the micrometer screw, 
to y q - 0 1 o o o The same operation is carried out with each of the two measuring 
headstocks, one at either end of a diameter, and the sum of the readings is compared 
with the sum of similar readings on a gauge bar of known length. For convenience, 
this gauge bar is mounted on the machine during the measurements, so that by simply 
traversing the saddle from the coil to the gauge the zero reading can be checked at 
any stage of the work. To provide for this, one of the cone centres is made specially 
large, and pierced behind the cone by a hole through which the gauge can be passed. 
In the original design the control of the vertical lever carrying the level was effected 
