MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 419 
A current passing from the spring to the wheel would thus be made GO times, and 
broken 60 times, in each revolution of the engine. 
Ihe engine, which is by Apps, is of particularly good construction for high speed 
work. Two horse-shoe electro-magnets are placed with their poles facing each other. 
One is fixed and the other revolves round an axis which is parallel to the cores and 
half way between them. The current in the fixed magnet has a constant direction, 
that in the revolving magnet is reversed every half revolution. The commutator is so 
arranged that the force between poles which are approaching each other is always 
attractive, and that between receding poles repulsive. Two screws regulate the 
pressure of the commutator springs. 
With a little care as to the adjustment, it was found possible to drive this engine at 
a speed of 100 revolutions a second. 
The secondary reversing engine consists of a square ebonite plate, near the corners 
of which are four holes filled with mercury. Upon it is supported an oscillating frame, 
which by dipping into the right and left hand pair of cups alternately, reverses at 
each oscillation any current passing through the instrument. The oscillations of the 
frame were produced by a rod going from it to a crank in the axis of a magnetic 
engine. The engine was similar to that of the wheel-break, except that as it was not 
driven at so high a speed, the screw adjustments of the springs were dispensed with. 
To regulate the speed, a friction-brake was used, consisting of a loop of silk which 
passed round a pulley on the axis, and was connected by an india-rubber band to a cord. 
This cord could be tightened by turning a handle round which it was wound. 
About 30 reversals per second was a sufficient speed for the purpose for which the 
engine was employed, and about the maximum which could be used without splashing 
the mercury from the cups. 
The batteries. —Ten small Leclanche cells, in series, worked the coil, four pint 
Grove’s the wheel-break, and two pint Grove’s the reversing engine. 
When the wheel-break was driven at full speed, the coil gave a spark about inch 
in length, the current being reversed 120 times in each revolution, or 12,000 times 
per second. 
Comparison of make and break currents. 
To test the equality of the make and break currents, the discharge was passed 
through a small vacuum tube. When the wheel-break tv as turned slowly the usual 
difference in the illuminations of the ends of the tube was observed, which difference 
was reversed on reversing the primary current of the coil. On working the break at high 
speed this difference disappeared, and no effect was produced by reversing the primary. 
This shows that at high speed the sums of the make and break currents in opposite 
directions were approximately equal. 
The secondary reversing engine was, however, used to guard against any difference 
in the above sums which might be produced by difference of leakage, &c., by reversing 
the secondary current about 30 times per second. 
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