ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 
consequence of variations in the thickness of the triple silk covering, there was a 
difficulty in getting exactly 18 turns into each layer. In the eleventh layer of A it 
was necessary to he content with 17 turns, and to place an extra turn on the outside, 
so as to form the commencement of a seventeenth layer—a circumstance which of 
course was taken into account in calculating the mean. The number thus arrived at, 
after correction for the thickness of tape, is the mean outside circumference. What 
we require is the mean circumference of the axis of the wire; it may be derived from 
the first by subtraction of half the difference between the tape readings for the first 
layer, and for the bottom of the gun-metal groove. 
The results obtained by Dr. Schuster, and myself when the coils were wound are : 
Mean of readings in millims. 
Correction for tape . 
Coil A. Coil B. 
1489*3 1487-5 
•6 -6 
Mean outside circumference . 
Correction for thickness of wire 
1488-7 
3-4 
1486-9 
3-4 
Mean circumference .... 
Mean radius. 
Mean circumference of A and B 
Mean radius of A and B (a) 
1485-3 1483-5 
236-39 236-11 
1484-4 
236-25 
Axial dimension of section in millims. . 19-9 19‘9 
Badial. 15*9 15 "4 
Distance of mean planes (2b') ... . 65"95 
Two or three readings were taken of the circumference of every layer, and to 
prevent mistakes in the number of turns, the plan described by Maxwell,'" of 
simultaneously winding string on wooden rods, was followed. Without some such 
device, there is great risk of confusion. 
In estimating the degree of accuracy obtainable, we must remember that the 
circumference of each layer is measured before the outer layers are wound on; any 
change produced by the pressure of these outer layers is a source of error. We had 
already observed a tendency in the measurements to be less during the unwinding of 
a coil than during the winding, and we fully intended to remeasure the coil after the 
spinnings were completed. This was done on December 6, 1881, by Mrs. Sidgwjck 
and myself. As we expected, somewhat smaller readings (by about f millim.) were 
obtained for the circumference of the middle layers. The results were : 
* 1 Electricity and Magnetism,’ II., § 708. 
4 R 2 
