FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
155 
annealed copper wire owing to stretching, but the hard rolled copper tape could be 
wound without any tension, and did not undergo any change of length. This was 
verified by graduating the tape itself on a 50-foot comparator, the errors of which 
were known; then winding the coil for trial, and unwinding and measuring the tape 
again, which was found not to have changed in length by more than a tenth of a 
millimetre in each 50 feet. The tape was supported horizontally on the polished 
surface of the comparator, and measured under a tension of 6 kgr. Young’s modulus 
was determined for each section. The tape was wound on each coil in two lengths of 
19 turns each, starting at opposite ends of a diameter, with two thicknesses of 
para ffin ed paper between each turn, so that the insulation could be tested with absolute 
certainty at any time. There were 38 complete turns, and nearly 6000 cm. of tape, 
on each coil. The probable error of the measurement was less than 1 mm. on the 
whole length, fie., less than 1 part in 100,000. The cods could not he boiled in paraffin 
after winding, as this would subject the tape to uncertain strains owing to the 
contraction of the wax. It was found necessary to re-wind the coils two or three 
times with minor improvements before the insulation proved to be perfect. Finally, 
silk ribbon was adopted in place of paper.” 
The following is a quotation from the notes of Mr. R. O. King, containing the 
summary of his measurements :•—• 
“We have the following data 
Coil I. 
1 st winding. Total length = 2970'17 cm. 
Corrected for tension, it = 2969'90cm. # 
2 nd winding. Total length = 2969'97 cm. 
Corrected for tension, it = 2969'69cm. 
There are nineteen double turns so that the mean radius 
2969'90 + 2969'69 
2 x 19 x 27r 
= 24'8767 cm. 
Coil II. 
1 st winding. Total length = 2972'88 cm. 
Corrected for tension, it = 2972'60 cm. 
2 nd winding. Total length = 2973'20 cm. 
Corrected for tension, it = 2972'92 cm. 
* These tension corrections reduce the value of the length measured under a tension of 6 kilos, to its 
value when wound under no tension. 
x 2 
