English: Siemens electrodynamometer, circa 1911. F = Fixed coil, D = Movable coil, S = Spiral spring, T = Torsion head, MM = Mercury cups, I = Index needle. This instrument is a form of an electrodynamic ammeter, a device that takes advantage of the fact that if there be two parallel wires through which currents are passing, then these wires are drawn together if the currents are in the same direction and pressed apart if they are in opposite directions. The Siemens electrodynamometer has a fixed coil which is surrounded by another coil having its axis at right angles to that of the fixed coil. This second coil is suspended by a number of silk fobres, and to the coil is also attached a spiral spring the other end of which is fastened to a torsion head. If then the torsion head is twisted, the suspended coil experiences a torque and is displaced through and angle equal to that of the torsion head. The current can be passed into and out of the movable coil by permitting the ends of the coil to dip into two mercury cups. If a current is passed through the fixed coil and movable coil in series with one another, the movable coil tends to displace itself so as to bring the axes of the coils, which are normally at right angles, more into the same direction. This tendency can be resisted by giving a twist to thetorsion head and so applying to the movable coil through the spring a restoring torque, which opposes the torque due to the dynamic action of the currents. If then the torsion head is provided with an index needle, and also if the movable coil is provided with an indicating point, it is possible to measure the torsional angle through which the head must be twisted to bring the movable coil back to its zero position. In these circumstances, the torsional angle becomes a measure of the torque and therefore of the product of the strenghs of the currents in the two coils, that is to say, of the square of the strength of the current passing through the two coils if they are joined up in series. The instrument can therefore be graduated by passing through it known and measured continuous currents, and it then becomes available for use with either continuous or alternating currents. The instrument can be provided with a curve or table showing the current corresponding to each angular displacement of the torsion head. It has the disadvantage of not being direct reading when made in the usual form, but can easily by converted into a direct reading instrument by appropriately dividing the scale over which the index of the torsion head moves.