136 
MB. H. TOMLINSON ON THE INFLUENCE OF STRESS 
for converting the readings of the galvanometer into electromagnetic units was deter¬ 
mined to be 
y=-3158 (tan </>+ '00839<£ sec <j>) } 
where <f> equals the deflection of the needle in degrees, and y is the value of the 
current strength in absolute measure. 
The magnetizing coils. 
During a considerable part of these investigations two magnetizing coils were 
employed, of which descriptions are now given. 
The larger of the two coils, which will be designated as the coil A, was made of 
about 1000 feet of cotton-covered copper wire, -^tli of an inch in diameter; the whole 
piece was divided into seven equal portions, and these, after having been well soaked in 
melted paraffin wax, were placed side by side and bound together by tape wound 
spirally along the whole length of the compound strand thus formed. This strand, 
after a further soaking in paraffin, was wound on a stout glass tube having an internal 
diameter of 1 inch and a length of 13 inches. The glass tube was provided at its 
two extremities with discs of hard wood ^ inch thick and 6 inches in diameter, 
and surrounding the whole of the coil was a zinc cylinder concentric with the glass 
tube, which being closed at its two ends by discs of the same metal served when filled 
with water to keep the coil cool (fig. 19). The ends of each of the seven portions into 
which the whole wire was divided were connected with separate terminal screws so 
that the seven coils could be used either in “ series ” or in “ multiple arc.” When 
arranged in “ series,” in which form they were employed in this particular branch of 
the encpiiry, the total resistance of the coils at 15° C. was 4'464 ohms. The com¬ 
pound strand was distributed along a length of 30 centims. in layers of five deep, 
so that the total number of turns amounted to 2100. The inner diameter of the coil 
was 3 centims. and the outer 7 centims.; therefore the magnetizing force at the centre 
and the two extremities would be respectively 47r X cX 70 x '986 and 4vX c X 70 X '498 
absolute units, where c denotes the current strength, and the average force throughout 
the whole length would be 477 X c X 70 X '742. 
The smaller of the two coils, the coil B, was constructed as follows :—A thin tube 
of polished brass, with a slit running throughout its entire length, 1|- inch internal 
diameter and 4~ inches long, was covered with vulcanised caoutchouc to a depth of ygtli 
of an inch, and on this was wound 3 lbs. of cotton-coverecl copper wire, yg-th of an inch 
in diameter, followed by 3 lbs. of wire, -njth of an inch in diameter. Inside the first 
tube was placed a second of similar kind, 1 inch in internal diameter, and connected 
with rings of ebonite with the first at the two ends. The second tube was concentric 
with the first, and of the same length, so that between the inner tube and the outer 
there was interposed a layer of air nearly -g-th of an inch in thickness. This arrange¬ 
ment was employed to prevent the heat from the magnetizing coil reaching any wire 
