TRANSACTIONS OF SECTION A. 939 
11. On a new form of Galvanometer. 
By Professor James Buytu, M.A., F.R.S.E. 
The author described a method of measuring electric currents by means of an 
instrument whose scale is graduated to equal divisions, and whose deflections are, 
from the intrinsic nature of the instrument, proportional to the current strength 
through all ranges. 
In principle the instrument depends upon the mechanical action exerted by a 
magnetic field upon a movable conductor carrying a current. In construction it 
is essentially similar to the well-known apparatus, due to Faraday, for showing the 
continuous rotation across the lines of magnetic force of a horizontal radial con- 
ductor carrying a current and having slipping contacts at its centre and circum- 
ference. The construction will be understood from the following description. 
Two bundles of permanently magnetised steel wires are made up in the form of 
cylinders having narrow axial holes. These are fixed with their axes in the same 
vertical line, separated by a narrow gap, and so that the north poles of the one set 
of magnets face the south poles of the other. In this gap the magnetic field is 
sensibly uniform and the lines of force vertical. Inserted in the gap isa thin 
circular disc of wood or vulcanite, having a central mercury cup and a concentric 
circular mercury trough at a short distance from it. A stout brass rod is provided, 
haying a short thick copper wire rigidly attached at right angles to its lower end. 
Its upper end passes freely through the axial hole in the upper magnet, and is 
rigidly attached to the lower end of a long fine torsion wire of steel or silver, 
whose upper end is fixed to a suitable support. The lower end of the brass rod 
dips into the central mercury cup, and the outer end of the copper wire (bent 
down a little at right angles) dips into the concentric canal of mercury. Stout 
copper wires are led from the central cup and canal to binding-screws suitably 
placed so as to form the terminals of the instrument. The upper end of the brass 
rod, which projects a little above the upper magnet, carries a long pointer, which 
moves over a horizontal circular disc, graduated either to degrees or to show 
amperes directly. This disc is fixed with the torsion wire passing freely through 
its centre. The whole is so enclosed as to be free from air current. 
The action of the instrument will be easily understood. Suppose a current sent 
through it. As is well known, the electromagnetic force acting on the radial 
current will tend to make it rotate in a horizontal plane with a uniform force. 
This rotation will go on till a certain angle is reached, when the moment due to the 
ead force is balanced by the moment due to the torsion of the wire. 
et 
2=the current strength, 
a=the length of the radial wire, 
N=the magnetic induction, 
A =the torsion constant, 
6 =the angle of equilibrium, 
we have 
, -_ 24 
4ia?N=AO; or i= a ae 
showing that the current strength is proportional to the angle of deflection. 
The instrument admits of several modifications. Two have been constructed 
in addition to that above described. In the one the permanent magnets are 
replaced by similarly placed electromagnetic coils, through which the current to 
be measured passes. In this case the square of the current strength is proportional 
to the angle of deflection. In the other the fixed magnets are dispensed with, and, 
instead, a eylindrical bundle of magnets is fixed coaxially to the brass rod, but 
insulated from it, so that it rotates along with the rod and radial wire. In this 
case the radial wire projects from the middle of the length of the magnet bundle, 
while the lower end of the brass rod is prolonged so as to dip into a mercury cup 
_ at a considerable distance below the lower end of the bundle. Owing to the 
influence of the exterior part of the circuit leading from the mercury canal to 
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