UNITS IN THE ELECTROMAGNETIC UNIT OF ELECTRICITY. 
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was placed on glass supports about 2 inches thick covered with a thin layer of 
paraffin, while each tray was insulated from the case by pieces of ebonite. 
The commutator was one which had been previously used by Lord Rayleigh, and 
had been designed by him. 
Fig. 6. 
The current from two Groves’ cells passes first through a tuning-fork interruptor, 
and then through the coils L M of an electromagnet. P N is a strip of brass with a 
piece of iron attached to it. When there is no current passing through the electro¬ 
magnet, the elasticity of the rod P N makes it press against a screw T, which is 
electrically connected with a binding screw R : when the current passes through the 
electromagnet the magnet attracts the iron attached to the rod P N and brings it 
into contact with the stop G, which is electrically connected with the binding screw S. 
The letters P, R, S indicate the same points in this figure as in fig. 1. All the places 
where contact is made -by the vibrating piece P N are covered with platinum, and the 
whole arrangement is fastened down to an ebonite board. As the current passes 
intermittently through the coils L N of the electromagnet, the vibrating piece P N 
strikes alternately against the parts G and T ; when it strikes against G the opposite 
plates of the condenser are connected with the two poles of the battery; when it 
strikes against T the condenser is discharged (see fig. 1). 
This commutator was found to work extremely well. When it was in good order 
the spot of light reflected from the mirror of the galvanometer through which the 
intermittent current passed never moved off one division of the scale, and the only 
thing by which the deflection could be distinguished from one due to a steady current 
was a slight indistinctness in the edge of the image of the spot of light. 
The speed of the tuning-fork interruptor was found by comparing it with that of 
the standard fork used by Lord Rayleigh in his determination of the ohm in absolute 
measure. The standard fork vibrates about 128 times per second, while the tuning- 
fork used in this investigation vibrates about 32 times per second. This fork was 
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