410 Description and Use of the Apparatus employed 
rent parallel to the axis of the spiral, and represented in inten- 
sity by the height of the coil, and the other by a circular cur- 
rent, represented by a section perpendicular to this axis in the 
(Cylindrical surface on which the spiral is coiled. And as the 
sum of the heights of all the coils is equal to that axis, it foL 
lows, that beside the action produced by the circular transverse 
currents, which are compared to those of the loadstone, the spi- 
ral produces at the same time the same action as a current of 
equal intensity along its axis. 
Now, if the conducting spiral wire returns along this axis, by 
inclosing it in a tube of glass placed within the spiral, so as to 
insulate the coils which compose it, the current of the rectilineal 
part of the wire being in an opposite direction to that which is 
equivalent to the part of the action of the spiral which is paral- 
lel to its axis, the first will repel what the second attracts, 
and attract what it repels. The longitudinal action of* the 
spiral will therefore be destroyed by that of the rectilineal por- 
tion of the conductor, and there will therefore remain only the 
action of the transverse circular currents, which are perfectly si- 
milar to that of a cylindrical loadstone. This reunion exists in 
the instrument shewn in Fig. 3. which presents exactly the ef- 
fects of a loadstone, and which will be a valuable instrument 
in electro-magnetic researches. When the coils have a great 
height, we have a conductor almost without power. 
The phenomena of the loadstone may also be exactly imitated 
by bending the conducting wire as in Fig. 4. where there i? be- 
tween all the portions of the conductor in the direction of the 
.axis the same compensation which takes place in the spirals al- 
ready mentioned, between the action of the rectilineal portion of 
the conductor^ and that which the coils exert in a contrary di- 
rection, parallel to the axis of the spiral. 
In the instrument Fig. 4. the brass-wire contained in the tube 
BH, is the prolongation of the one which forms the cii’cular 
rings E, F, G, &c. and each ring is connected with the pre- 
ceding one by a small arc of a spiral, of which eacli coil has a 
great height, in relation to the radius of the cylinder on which it 
is coiled. 
The action exerted by the projections parallel to the axis of 
the tube of those small spiral arcs marked by M, N, O, &c. be- 
