906 REPORT—1885. 
the mode of effecting the measurement was founded on Faraday’s law, according to 
which a ferro-magnetic mass placed in a variable magnetic field experiences forces 
tending to make it move from places of weaker to places of stronger force. The 
essential parts of the milliamperemeter are shown in the sketch diagram, fig. 1. It 
consists of an electro-magnetic coil, fixed with its axis vertical, and a little cylin- 
drical mass of soft iron hung from one end of a light balanced lever so as to be 
free to move up and down in a circular arc, deviating but little in its middle and 
at its two ends from the axis of the coil. 
The measurement is given by the deflections indicated on a scale by the end of 
Fig. 2. 
the balanced lever, when a weight of known amount is hung on the ring below the 
iron mass. To screen the iron from the eflects of the earth’s magnetism the coil is- 
enclosed in an iron box. 
In the hecto-amperemeter the variable magnetic field is obtained by a suitable 
disposition of the metallic conductor conveying the current to be measured. The 
conductor may he taken as consisting of two thick copper plates, shaped each 
according to the sketch, fig. 2, supported in a vertical position parallel to each. 
other, say one centimetre apart, and metallically connected at the place indicated 
by B. At A is fixed a suitable electrode. The course of the current is therefore 
from A to B, and from B across to and through the other plate to the part of it 
corresponding to A, which forms the other electrode. In this way, two similarly 
varying magnetic fields are produced, and the balanced lever, capable of motion 
in a plane situated midway between the plates, carries two masses of iron, one in 
each field. In other respects, the instrument is similar to the milliamperemeter. 
The electrometer consists of an air condenser with one of its plates capable of 
a to-and-fro motion so as to vary the capacity of the condenser. 
The fixed brass plates are supported so as to be accurately parallel to each other 
and in metallic connection, while they are thoroughly insulated from the case of 
the instrument. The movable plate is of aluminium, and is supported in a vertical 
position on a knife edge; the plane of its motion being parallel to the fixed plates. 
and situated midway between them. The upper end of this movable plate has a 
fine prolongation which serves as a pointer for indicating the deflections on the 
scale of the instrument, and at its lower end is fixed a knife edge having its length 
perpendicular to the plane in which the plate moves, 
When the fixed and movable plates are connected respectively to two points of 
an electric circuit between which there exists a difference of potential, the movable: 
plate tends to move so as to augment the electrostatic capacity of the instrument,. 
and the magnitude of the force concerned in any measurement is proportional to 
the square of the difference of potential by which it is produced. In the use of 
the instrument this force of attraction is balanced by the horizontal component of 
a weight of any convenient amount hung on the knife edge at the bottom of the 
movable plate. 
