ELECTRODYNAMIC QUALITIES OF METALS. 
57 
and contractions when the weights were applied and removed. In some of the experi¬ 
ments with “ons” and “offs” of 14 lbs., about 90 centims. of the wire were heated 
to 100° C. by a stream of hot water (as described in § 201), but this left no per¬ 
manent change in the wire. 
201. The accompanying diagram (I.) shows the arrangement of the apparatus by 
means of which the results were obtained. The wire, W, experimented on was attached 
to a fixed support near the ceiling of the laboratory, and hung vertically downward, 
passing along the common axis of the magnetizing and induction coils, F, and was 
kept stretched by the scale-pan, D, which, as stated above, weighed exactly 1 lb. The 
coils, F, were supported on a clamp attached to the wire at then- lower end, as shown 
in the drawing; and thus, as the length of the induction coil was small compared 
with the total length of the wire, motion of the wire relatively to the induction coil 
due to the stretching produced by the applied stress, was in great measure avoided. 
The magnetizing coil was 86 centims. long, and was composed of two layers of silk- 
covered copper wire. The inner layer contained 26‘7 metres of wire, of No. 23 
B.W.G., arranged in a solenoid'" of 960 turns, the outer layer, 24’3 metres of No. 20 
wire, arranged in a solenoid of 728 turns. The resistance per metre of these wires was 
'0673 and - 0522 ohm respectively, and the total resistance of the coil after it was 
wound was 3T34 ohms. The total length of the induction coil, which was contained 
within the magnetizing coil, was 31'5 centims. It was made up of two layers of silk- 
covered copper wire, of No. 29 B.W.G., laid on in 1439 turns. The wire thus coiled 
on was 10 metres long, and had a resistance of T84' ohm per metre, and the total 
resistance of the coil, including 1 metre of electrodes, was 2'204 ohms. 
The magnetizing coil was wound on the outside of a compound tube, made up of two 
tubes of thin brass, of different diameters, placed one within the other with their axes 
coincident, the external diameter of the inner tube being less than the internal dia¬ 
meter of the outer by about 3 millims. The induction coil, which was wound on a 
thin copper tube just fitting the wire experimented on, was enclosed within the inner 
brass tube in such a position that its ends were at equal distances from the extremities 
of the magnetizing coil. The space between the two brass tubes formed a channel 
* The common use of the word “ helix ” in this sense is utterly illogical. The idea of helix is not essential, 
but accidental, and in no practical case is it of any consequence whether it is a right-handed or a left- 
handed helix. There is nothing of helical quality in a cylindrical tube composed of two metals in two 
parts of its circumference, with the junctions of these metals kept at unequal temperatures. The thermo¬ 
electric current round the circumference of the solenoid produces the kind of magnetizing influence which 
is commonly produced by a helix, and constitutes precisely the arrangement which Ampere called a 
solenoid. It is only because the ordinary helix, with electric current flowing through it, produces more 
or less approximately (very approximately indeed in the case of a helix with many turns) the same effect, 
that it is available for the electro-magnetic uses. It seems desirable, therefore, to take advantage of 
Ampere’s original word “solenoid,” and, except in cases in which the helical quality is taken into account, 
to give up the name “helix.” The electro-magnetic solenoid may also be called a bar electro-magnet 
without soft iron core. 
MDCCCLXXIX. 
I 
