530 
ME. Gr. GOEE ON ELECTEOTOESION. 
screw attached to its upper end and fixed to the top of the wooden support. To the 
lower and free end of the bar was fixed by a screw a movable and rigid horizontal 
pointer of brass, 47 centims. long, weighing about 300 grammes (=10^ ounces), and 
balanced by an adjustable counterpoise. In a few experiments, where rapid and 
continuous vibration was required, the free end of the pointer was tipped with a thick 
piece of platinum wire. 
With this apparatus the torsional movements were sufficiently large not to require 
the employment of the method of detecting and measuring them by means of a mirror 
and telescope, or luminous image reflected to a distant graduated scale. The lower end 
of the iron rod had fixed to it a vertical piece of thick platinum wire about 3 centims. 
long, which dipped into mercury contained in a little platinum cup separately supported 
by a horizontal rod of copper to act as a conductor ; or it had sometimes attached to it 
by means of a screw, B (fig. 2), a strong loop of copper, A, for the purpose of supporting 
a weight, and provided with a stout platinum wire (C) to dip into the mercury. Instead 
of the axial bar, a wire of iron, about 2 mm. thick, was generally employed, and the 
torsions obtained with it were much greater. The helix was excited by means of 6 or 
12 Grove’s elements, each of about half a litre (or one pint) capacity, and having 
platinum plates 10 centims. long and 8 centims. wide. 
The current from 4 or 6 of those cells, in double series, being now passed in an unin- 
terrupted state through the coil, and simultaneously with it, or during its passage, the 
one from the remaining 6 or 8 cells, in double row, being passed through the bar or 
wire, the index moved either to the right hand or the left, according to the directions 
of the currents. 
By fixing a platinum-tipped horizontal brass screw upon a separate support, so that 
the screw just touched the side of the platinum end of the pointer, and completing the 
axial circuit through it, continued vibration of the pointer was obtained, provided the 
screw was placed on the proper side and the index was sufficiently rigid. 
The more conspicuous effects may be successfully obtained with a much smaller 
apparatus. 
In all cases the torsion was accompanied by elongation, and the resulting motion was 
in a diagonal direction, and the maximum of vibration was therefore obtained by placing 
the contact-screw so as to touch the index between its side and upper surface. By 
placing the screw in such a position that the vertical movement alone did not break 
the contact, rapid vibration, due to torsion only, could be maintained during any length 
of time ; and by placing it in contact with the upper surface of the pointer and passing 
the current through the coil and index only, continued vibration, due entirely to elon- 
gation, could also be readily obtained. 
2. Does an axial electric current lengthen an iron bar 1 
To ascertain this I passed the current through the bar only ; but not even a tendency 
to vertical vibration could be perceived. I also tried a thinner rod, and a wire strained 
