64 
ME. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
20°*0 C. of the coils on cylinders Nos. 1 and 2 are 16'0018 cm. and 16*0013 5 cm. 
respectively; those on cylinders Nos. 3 and 4 are 16*0054 cm. and 16*0054 cm. 
respectively. The amplitude of the periodic variation dies away towards one end for 
the coils on cylinders Nos. 1 and 2 , but for the coils on cylinders Nos. 3 and 4 it does 
not. The graphs for the variations in diameter of the coils lend support to this 
division of the cylinders into pairs, and as the same lathe and the same portion of the 
leading screw were used for the turning of all, the differences appear to be due to a 
slight difference in the skill and touch of the late Mr. Taylerson, who turned 
cylinders Nos. 1 and 2 , and Mr. Tribe, who was responsible for cylinders Nos. 3 and 4. 
In all cases the turning is remarkably good. 
The mean axial length of the coils on one cylinder is obtained from the graph 
showing the periodic variation of the pitch and the corresponding graph for the invar 
metre. Limiting our attention to the coils on cylinder No. 1, it is clear that the 
extreme mean length of the coils is equal to the length of the invar metre section 
AB plus the length corresponding to the difference of the ordinates OA and PB. 
The length of the invar section is 15*9996 2 cm. and the difference of the ordinates 
corresponds to +17//. Hence the mean length of the coils on cylinder No. 1 is 
15*9996 2 + 17// = 16*0013 2 cm. at 14°*5 C. = 16*0018 cm. at 20°*0 C. 
Table VII. gives the mean axial length of the coils at 20°*0 C. 
Table VII. 
Coils on cylinder. 
Mean axial length at 20° • 0 C. 
cm. . 
1 
16-0018 
2 
16-0014 
3 
16-0054 
4 
16-0054 
These observations for the mean axial length were made in April, 1913. The 
results are identical within the limits of the errors of observation (about 3//) with 
those obtained in March, 1912. A change in the length of a coil of 20// produces a 
change in the mutual inductance of that coil and the two discs of 1 part in 100,000. 
Section 14.—Erecting and Adjusting the Instrument. 
Before assembling the parts of the machine, a concrete block, built up of Keene’s 
cement and Thames ballast, was prepared as a foundation. The block is non-magnetic, 
is 8 m. long, 80 cm. wide, and 120 cm. deep. Slide rails of gunmetal are bolted in 
position on this block and to secure greater stability the rails are sunk 5 cm. into the 
concrete. The rails are in three pairs ; one pair support the motor ; a second pair the 
fly-wheel; and the third pair, which are nearly 4 m. long, support the portions with 
the rotating discs. A period of twelve months was allowed for the concrete block to 
