OF ELECTEOSTATIC UNITS IN THE ELECTEOMAGNETIC UNIT. 
411 
The New Absolute Electrometer is identical in principle with the Absolute Electrometer 
described in the Report of the British Association, 1867. The principal improvements 
in the new instrument are the introduction of a new sighting arrangement, by which 
errors from parallax are avoided, the substitution of a metal disk supported by springs 
for the disk balanced by a counterpoise, and the addition of an idiostatic gauge and a 
replenisher, by means of which the charged disk can be maintained at a constant potential. 
By this instrument the force of attraction between two disks of known area at a constant 
difference of potentials is measured in grammes weight. The difference of potentials 
between the two poles of a battery can thus be measured, and the area, force, and distance 
being known, can be expressed in absolute electrostatic units of space, time, and mass. 
In these measurements the units employed were : — for unit of mass the gramme, for unit 
of time the second, and for unit of space the centimetre. 
The strength of the current was measured by the force acting between two portions 
of itself in the circuit of an electrodynamometer (see Plate XXXII. figs. 1 & 2). This 
circuit consisted of three coils, two large fixed coils (a, b), and a third smaller coil ( c ) 
movable round a vertical axis. The larger coils contained each upwards of 3700 metres 
of fine silk-covered wire wound upon brass rings, 30 centims. in diameter, with parallel 
flanges about centims. broad. 
The two coils contained on an average 3700 turns of wire each, the depth of the coils 
being somewhat less than 2 centims. and the breadth of the rings being 2 centims. One 
end of the wire in each, before winding, was soldered to the ring, the ring thus forming 
one of the terminals of the coil. The two coils were placed on an oblong wooden 
pedestal (li) (supported on three screws) with their planes vertical and parallel to one 
another, the line joining their centres being perpendicular to the plane of each coil. 
The rings were fitted to wooden concave supports ( g , g'), to which they were secured by 
two bars of wood (f, f) resting across the rings on the inside, and firmly attached by 
means of screws to the wooden stand. The parallelism of the coils was preserved, as far 
as possible, by means of stout hard-wood bars of equal length (?') inserted at intervals 
between the rings perpendicular to the planes of the coils. To secure the most uniform 
magnetic field in the neighbourhood of the coils, the Helmholtz arrangement of the coils 
was chosen, the distance from centre to centre of the coils being approximately equal to 
their radius. 
The movable or suspended coil ( c ) consisted of upwards of 900 metres of finer wire. 
The brass ring upon which it was wound was 6| centims. in diameter, and had flanges 
2% centims. broad. It contained upwards of 3000 turns of wire, the depth of the coil 
being 2 centims. and its thickness 1^ centim. 
This movable coil, which was suspended between the two fixed coils, was protected 
from disturbing currents of air by a wooden box (k) which stood between the two fixed 
coils, being fastened to the stand on which they rested. On one side perpendicular to 
the planes of the coils this box was left open ; and after the coil had been suspended, a 
glass screen was introduced. Subsequently, to prevent any error from distortion of the 
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