RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
61 
Measurement of the Axial Lengths of the Coils and the Variation of Pitch. 
The pitch measuring machine, also constructed by Sir W. G. Armstrong, Whitworth 
and Co., consists of a straight horizontal bed provided with dead centres on which the 
mandrel carrying the coil can be mounted. Parallel to the line of centres is a slide, 
along which a small slide rest can be traversed by means of a calibrated measuring 
screw, of T \j-inch pitch. The measuring wheel which actuates this screw can be read 
to ^ UU ] , )(H ; inch by means of a vernier, but the accuracy of repetition of readings is 
somewhat less than this. The slide carries a microscope whose optical axis intersects 
the line of centres of the machine at right angles. The microscope is focussed on the 
coil and is moved by means of the measuring screw so as to bring the consecutive 
turns of the coil, one after the other, into a definite position with respect to the cross 
wires in the eye-piece, the reading of the measuring wheel being taken at each setting. 
The calibration of the screw is effected by focussing a graduated line standard in the 
microscope and taking similar readings on the graduations of the bar. The measuring 
screw and slide are themselves movable on a second slide in a direction parallel to the 
axis of the microscope, so that by withdrawing them slightly the standard bar can be 
inserted at any time between the coil and the microscope, and check readings taken 
on it without disturbing the coil. 
The standard employed was an invar metre, the history of which is so well known 
that the probable error over a length of 12 cm. (the axial length of a coil) is less 
than 1 fj.. 
For the measurements on each cylinder, readings on successive half centimetre 
divisions of the invar metre were taken over a length of 14 cm., and readings over a 
length of 12. cm. (corresponding to the axial length of the coils), were frequently 
made. Any change in temperature of the screw of the measuring machine could 
thus be allowed for. 
As already explained, the two wires on each cylinder are wound in double screw 
cut grooves. To measure the mean variation in pitch of the two coils it is better, 
therefore, to make observations on the spaces between neighbouring wires than on the 
wires themselves, for in the latter case twice as many observations are required. In 
practice, observations on the spaces proved the easier way ; the white marble showed 
up well between the wires, and it was not difficult to bisect the white spaces by means 
of the cross wires of the microscope. In addition, check observations on the wires 
were always made. For a complete set of measurements 96 readings are required, 
and three such sets in different axial planes were made on each cylinder. 
After correcting the readings, the mean pitch of the coils was calculated within a 
few parts in 10,000 by dividing the difference of the extreme readings bj 7 the number 
of turns. The approximate pitch thus obtained was 0'065635 inch (= 0'166712 5 cm.). 
We multiplied this number by 1, 2, 3, 4, ..., &c., up to 95, and from the resulting 
products the corresponding pitch readings of a coil were subtracted. If the coil had 
