136 I)RS. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
work so considerable compared with, the power of the motor that it was not feasible 
to use an eddy-current brake to give further stability. 
(2) This apparatus (fig. 4) was made as light as possible so as to be easily portable. 
Fixed Lens 
The shaft of a Siemens-Schiickert 12-volt, ^ h.p. motor was lengthened and fitted 
with a couple of ball-bearings carried by brackets from a rigid base-plate. No fly¬ 
wheel was used, the required steadiness being given by an eddy-current brake consisting 
of an aluminium disc (radius = 7 cm.) attached to the shaft and spinning between the 
four poles (distant 5^ cm. from the axis) of an electromagnet excited by a constant 
current. In this case the friction of the bearings only formed a small fraction of the 
total work done. 
The different circuits were excited as follows :— 
Armature.6 volts, 0-3 amperes. 
Field.6 ,, 3 
Brake.2 ,, 1 ampere. 
Except in special cases the whole range of frequency could be covered by variation 
of the armature current alone. 
In addition to portability (the total weight was 8 kgm.) this apparatus possessed, 
on account of its small inertia, the advantage of extreme rapidity in attaining a steady 
speed. For a given adjustment the final speed was reached in about 3 seconds from 
rest, whereas the former apparatus required 10 minutes. 
The chief source of trouble in obtaining a constant speed was found to be due to 
irregular variations in brush-contact on the commutator of the motor. Both carbon 
and solid copper brushes were found to be unsatisfactory on this account. These were 
