84 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
current in both coils on any one cylinder, and the plug board allows of the reversal of 
the current in any one or more of the helices. Each helix is designated by a number 
and a letter which are marked on an ebonite bridge at the top of the multiple 
commutator ; the turning heads are also numbered to enable changes to be rapidly 
made without likelihood of error. The lower commutator reverses the current in all 
of the coils. 
By suitable conducting straps the coils on one cylinder may be placed in parallel 
with those on another cylinder. In many of our observations the coils on cylinders 
1 and 2 were placed in parallel with the coils on cylinders 3 and 4. 
( c) The Reversing Switch. —Preliminary experiments indicated that the make and 
break of the current through the coils had a considerable inductive action on the 
galvanometer circuit. The deflection of the galvanometer thus produced amounted 
at times to 50 cm., on a scale at a distance of 2 m., and prohibited such rapid 
reversal of the current as we desired to make. Similar inductive action is remarked 
on by Prof. J. V. Jones in the ‘Report of the British Association for 1890.’ 
Had the galvanometer circuit been a stationary one, we might have introduced a 
compensating system, but this was not possible with the system of rotating conductors 
we had installed. It was evident that the galvanometer circuit should be broken 
before making or breaking the current circuit, and as the position of rest of the 
galvanometer coil on open circuit is, in general, different from that when the circuit is 
closed, it appeared desirable to have the galvanometer excessively damped when not 
included in the main circuit. The switch shown in fig. 19 enables this to be done. 
All the connections are shown in the figure and it is not necessary to describe the 
switch in detail. On moving from stud 1 to stud 2 the galvanometer circuit is 
shunted by a negligible resistance and the main galvanometer circuit is broken. On 
moving to stud 3 the current is broken—on to stud 4 the current is made again but 
in the reverse direction—and when the movement is continued to stud 5 the 
galvanometer is again placed in the brush circuit. This switch proved very 
convenient in practice and greatly facilitated our work. 
(d) In addition to the commutators already described, a simple commutator was 
added to reverse the potential leads attached to the standard resistance. Such 
reversals were made to eliminate electrostatic effects. 
(e) Galvanometer. —This was of the Ayrton-Mather type and was very kindly lent 
to us for the work by The Cambridge Scientific Instrument Company, Limited. The 
resistance of the galvanometer is 16 ‘5 ohms and the period of the coil is 5'2 seconds. 
At a distance of 2000 scale divisions the sensitiveness is 57 divisions per microvolt. 
The external resistance for aperiodic working is 50 ohms. 
When the current through the eight coils of the Lorenz apparatus is 2 amperes 
and a direct measurement of a resistance of O'Ol ohm is in progress, the difference of 
potential on the standard resistance is 0'02 volts. In making the measurements the 
current is reversed, and on reversal there is a change in the rest point of the 
