tions are known to the Lords of the Admiralty, and it is understood that they 
will before long* be subjected to competition with the system of other inventors 
before anything is fixed in regard to naval range-finding. 
I. General Principle of the Instruments. 
The general principle is an ingenious application of Wheatsone’s Bridge, 1 and 
consists in determining at a distance the angle of inclination to one another of 
two needles fixed at points widely separated, or, on the other hand, in making the 
two needles parallel. For that 
purpose, two metallic needles, L 
and L' are pivoted on the vertical 
axes A and B. Their free ex¬ 
tremities are in contact with two 
metallic arcs of a circle, h and h\ 
which are attached to one another 
by two weak conductors ab and cd. 
These conductors, are attached to 
one another by a transverse wire 
pp> of the same conductivity, 
which in its turn is surmounted 
by a dead-beat 2 galvanometer g. 
The pivots of the needles are 
united respectively to two poles 
of the accumulator E. The cur¬ 
rent, on arriving at one of the 
pivots, follows the corresponding 
needle and divides itself on the 
arc into two parts which reunite on the other needle. This circuit constitutes 
therefore a Wheatstone’s Bridge, in which the resistances of the four portions 
vary with the deflection of the needles, since every movement of one of the latter 
introduces into the circuit a greater or less portion of the corresponding arc. 
When the needles are parallel their extremities touch homologous points on 
the two arcs, and each circuit derived from those points, on one and the other 
i Wheatstone’s Bridge is theoretically repre¬ 
sented in this figure, where E is the battery and 
G a galvanometer. If r 1 , r 2 , r 3 and r 4 represent 
respectively the resistances of the wires or coils AB, 
BC, AD and DC of the Bridge, and if these resis¬ 
tances he such that no current passes through the 
galvanometer in the direction BD, we have the 
equation: 
r l r 4 = r 2 r 3 
Let us, then, so arrange matters as to have two 
standard resistance coils in AB and AD, of which 
we know the ratio, and a coil of variable resistance 
in BC. Then vary the resistance of the latter until 
no current passes through BD, when we shall be 
able to measure the resistance of CD, the quantity 
required, by 
r 4 ——r 2 
r \ 
2 A dead-beat galvanometer or ammeter, as it is frequently called, is one with a very light needle 
moving m a very powerful magnetic field. The moment of inertia of the light needle being very 
small, its oscillations are very quick and die out very rapidly, so that if the current that is being 
measured has a sudden change in its strength, the needle moves sharply from one point of the 
scale to another point, where it stops dead, or dead-beat. This decisive action of galvanometers 
with light needles renders them particularly suitable for artillery purposes. A dead-beat galvano¬ 
meter has moreover the advantage of giving correct reading in any position, provided that its 
needle be well balanced.— F.F.B.L. 
Fig. 2. 
