NON-SPARKING KEY.


Profs. Ayrton and Perry lately described and exhibited before the
Physical Society their new ammeters and voltmeters, also a non-sparking
key. The well known ammeters and voltmeters of the authors used for
electric light work are now constructed so as to dispense with
a constant, and give the readings in amperes and volts without
calculation. This is effected by constructing the instruments so
that there is a falling off in the controlling magnetic field, and a
considerable increase in the deflecting magnetic field. The deflections
are thus made proportional to the current or E.M.F. measured. The
ingenious device of a core or soft iron pole-piece, adjustable between
the poles of the horseshoe magnet, is used for this purpose. By means of
an ammeter and voltmeter used conjointly, the resistance of part of the
circuit, say a lamp or heated wire, can be got by Ohm's law. Profs.
Ayrton and Perry's non-sparking key is designed to prevent sparking with
large currents. It acts by introducing a series of resistance coils
determined experimentally one after the other in circuit, thereby
cutting off the spark.

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NEW INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS AND ELECTRO-MOTIVE
FORCE.

By Messrs. R. E. CROMPTON and GISBERT KAPP.