372 
MESSRS. W. E. AYRTON AND H. KILCIOUR ON THE 
edition of this book has appeared, and, in consequence of a suggestion made to 
Professor Everett, the word “balls” has been added after the word “copper” in 
this new edition, as well as the following paragraph :— 
“ Influence of Size. 
“ According to Professor Ayrton, who quotes a table in ‘ Box on Heat,’ the 
coefficient of emission increases as the size of the emitting body diminishes, and for a 
blackened sphere of radius r cm. may be stated as 
“ 0-0004928 + 
0-0003609 
r 
“ The value in Macfarlane’s experiments was 2.”] 
The laws which govern the loss of heat from thin cylindrical conductors have not 
only considerable scientific interest in showing how the shape of a body affects the 
convection currents, but they are of especial importance to the electrical engineer in 
connection with glow-lamps, hot-wire-voltmeters, fuses, &c. We, therefore, thought 
it desirable to ascertain the way in which the law of cooling for thick wires, which 
involved the diameter raised to the power three-halves, passed into the law for the 
cooling of thin wires, involving only the first power of the diameter. For this object 
the investigation described in the present communication was commenced at the 
beginning- of 1888. 
A considerable number of preliminary experiments having been conducted for the 
purpose of arriving at the best conditions to be adopted in the investigation, we 
finally, in April, 1888, decided to measure the emissivity at different temperatures 
from nine platinum wires, having respectively the diameters of 1, 2, 3, 4, 6, 8, 10, 12, 
and 15 mils, or thousandths of an inch. Wires of these sizes having been ordered 
from Messrs. Johnson, Matthey, and Co., they were found, when received, to have 
the following diameters at 15° C. : — 
Mils. 
Millimetres. 
1-2 
0-031 
2-0 
0-051 
2-9 
0-074 
4-0 
0-102 
6-0 
0152 
81 
0-206 
9-3 
0-236 
111 
0-282 
14-0 
0-356 
Throughout this paper we have given the diameters of the wires both in mils and 
in millimetres. Tt may seem unscientific to mix up dimensions in thousandths of an 
inch with dimensions in centimetres, but, while it is convenient, for the purposes of 
comparison, to use one square centimetre as the unit of area in experiments on 
