RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
69 
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measurements. Reference marks are made on the flanges of the cylinders, and when 
the coils are parallel the mean distance between two such marks, one on each cylinder, 
in the two positions of the coils, is exactly equal to the mean distance between the 
mean planes of the coils. If there are two marks diametrally opposite on each cylinder 
and two distances are measured for each position of the coils, the mean distance between 
the marks is the mean distance between the centres of the coils if the latter are 
approximately parallel. In our case a want of parallelism of 0'25 degree (far in excess of 
that met with in practice) introduces an error of less than 1/x in the determination of 
the distance between the centres of the coils. In practice we have four marks on 
each cylinder, these being at approximately equal distances from the mean plane of 
a coil, and the distance of each mark from such plane is known within 10/x or 20^. 
The vertical distances of the marks from the axis 
of the coil are the same within 0T mm., and for 
the measurements under consideration the dis¬ 
tances may be regarded as identical. 
Fig. 15 represents diagrammatically two cylin¬ 
ders at a mean distance 00' apart. When the 
coils are parallel, the distances ac, eg , bd, and fh 
are approximately equal to 00'. When the coils 
are reversed but not interchanged, the distances 
are in general different from what they were before ; 
let the distances be a'c', e'g’, &c. With parallel 
coils each of the mean distances (ac + a'c')/2, (eg + e'g')/2, &c„, is equal to the average 
distance apart of the mean planes of the coils. If the two coils are not parallel but 
very nearly so, the average of the distances ac, a'c', eg and e'g' is equal to the average 
of the remaining four distances and also equal to the average distance apart of the 
centres of the coils. 
In the flanges of each cylinder four brass plugs are screwed and cemented in 
position. The plugs in each flange are at opposite ends of a diameter, and the four 
plugs in one cylinder are contained in a common axial plane. Fig. 4 shows two of the 
plugs ab in cylinder No. 3, and two cd in cylinder No. 4. On the faces of the plugs 
thin rectangular pieces of platinum are soldered ; the surfaces of these are polished, 
and a scale of half millimetres, cut at right angles by two horizontal lines, half a 
millimetre apart, is engraved on each piece. The scales were engraved by the late 
Mr. Donaldson of the Metrology Department. 
For the measurement of the distances, two microscopes are mounted on a special 
platform supported on a tripod, the base of which can be clamped to the bed of the 
Lorenz machine by means of the wing nuts w, tv', tv", &c., shown in fig. 4, and by 
means of screws the platform can be given the necessary movements to enable both 
microscopes to be rapidly focussed without any change in their relative position. 
When the platform is clamped by the wing nuts ww', the lines on the plugs a and c 
Fig. 15. 
