ME. G. GOEE ON ELECTEOTOESION. 
531 
by a weight, and applied the contact-screw to the under side of the index (as well as to 
the upper), in order to detect any shortening, but obtained no positive results ; probably 
an extremely minute degree of shortening occurred, and was counterbalanced by ex- 
pansion produced by heat of conduction-resistance. 
3. Methods and conditions of obtaining the torsions. 
The torsions may be produced: — (1st) by the passage of axial currents alternately in 
opposite directions, (2nd) by the alternate passage of coil-currents and axial ones, 
(3rd) by the simultaneous passage of both, and (4th) by the temporary passage of an 
axial current during the continuance of a coil one, or vice versa. The 1st and 2nd 
methods yield only very small torsions, and the 3rd and 4th produce exceedingly large 
ones. 
The best conditions are : — a slender wire of the softest iron, with but little weight 
attached to it, and free from mechanical twist, enclosed nearly its entire length in a 
powerful voltaic coil ; a downward current in the iron wire simultaneous with a coil- 
current producing a north * pole below ; and repeatedly applying the pair of currents 
synchronously with the movements of the index (the two currents may be made 
simultaneous in their action by causing them either to leave the two batteries or return 
to them by a single wire). Or better, a continuous current in the coil, producing a 
south pole below and a temporary one in the axial wire, and reversing the direction of 
the latter synchronously with each half-vibration of the pointer: a very convenient 
plan is to pass the entire current through the coil, and during its continuance divert, 
by means of a reverser, such portion of the current as will flow through the axial wire, 
and reverse the direction of that portion at the end of each half- vibration. A quantity 
of electricity passing through the axial wire of about four times that which circulates 
through the coil is a suitable proportion. 
With a soft-iron wire 2 - 6 metres long and T75 mm. thick, four Gkove’s cells 
being arranged as two attached to the coil, and eight as four to the axial wire, a single 
contact produced a movement of 23 mm. of the end of the pointer, and by synchronous 
contacts a swing of 80 centims. (or 31 inches=102 degrees), or more than a quarter of a 
circle, was easily obtained. [With a shorter and thinner wire and suitable helix, and 
the current of a Noe’s thermopile of 80 elements connected as 20x4, a swing of 36 
inches was produced.] The torsional movements, when first observed, were so small 
as to require the aid of a magnifying-glass in order to detect them ; they are all 
attended by audible sounds in the iron. 
The currents employed in these experiments being large and of low tension, the 
following reverser offering but little resistance was employed. Upon a wooden base 
were fixed four binding-screws, A, B, C, and D (Plate XL1I. fig. 3). To A and B 
were fixed two flat and very flexible brass springs (E and E), united by a cross piece of 
* Throughout this paper I call that a north pole which points to the south, and that a south pole which 
seeks the north. 
4 b 2 
