318 
LORD RAYLEIGH AND MRS. H. SIDGWICR ON THE ABSOLUTE 
number of parallel soft iron armatures, disposed symmetrically round tlie circumference. 
In the course of the revolution these armatures pass in succession between the poles 
of a vertical horse-shoe electromagnet, so as almost to complete the magnetic circuit. 
It is much better that the armatures should pass between the poles than over them, as 
in the most usual arrangement, for in the latter case the bearings are subjected to an 
unnecessary and prejudicial strain. The wheel may be used either with or without an 
independent driving power. In the former case the power should be very steady, and 
adjusted so as to give by itself nearly the speed intended. The currents from the 
interrupter-fork are passed also through the electromagnet of the engine, and give the 
force required to accelerate or retard the motion so that it may exactly synchronise 
with the fork, one armature passing for each complete vibration. If the independent 
power is in excess, the phase of the motion is such that the electromagnet is excited 
principally after the armatures have passed through the electromagnet; if the inde¬ 
pendent power is in defect, the electromagnet is excited principally while the armatures 
are approaching it. Within certain limits any necessary acceleration or retardation is 
obtained by suitable self-acting adjustment of phase. 
If when the wheel is moving steadily under the influence of the intermittent 
currents, a slight disturbance is communicated to it, oscillations will set in, the wheel 
being alternately in advance and in the rear of its proper position. In some cases 
these oscillations are very persistent, and interfere seriously with the utility of the 
instrument. To check them, a hollow ring filled with water is attached to the shaft, 
and revolves with it. When the rotation is perfectly regular, the water behaves as it 
it were a rigid body and offers no impediment to the motion, but it tends to check 
variations of speed of moderate period. The oscillations, when they exist, are usually 
audible ; and in any case the behaviour of the wheel in this and other respects may be 
examined by looking at the interrupter-fork through a paper disc carried by the wheel 
