1872.) Electricity. 407 
duction of conductors of known and unknown resistances. In the ordinary 
use of the apparatus, the gaps A and pD are closed, either with thick copper 
latches of insensible resistance, or by wires, the resistances of which are 
known in comparison with that of the standard wire, EF. The resistances to 
be compared are inserted at B and c. The battery wires are connected with 
binding screws at g and h, one terminal of the galvanometer being connected 
to the screw, k, the other galvanometer contact being in connection with a 
sliding contact, which is moved backwards and forwards on EF, until the posi- 
tion is found which causes no current to pass through the galvanometer. It is 
then clear that the scale reading corresponding to this position would in general 
indicate the true ratio of the resistances at B and c, only when the resistance of 
the copper band is an insensible fraction of these resistances as well as of the 
resistance of the wire, EF. But if the resistance to be measured is less than 
that of the whole metre wire, a measurement of it in terms of the latter, which 
is quite unaffected by the resistance of the connections, can be made by 
obtaining the balance by the sliding conta& on EF; first, when the unknown 
resistance is inserted at p, and a conneétor of insensible resistance at A; and 
secondly, when the two have been interchanged, the gaps B and c being occu- 
pied by any convenient conductors which do not differ more in relative resist- 
ance than the wire to be measured, and the whole of the wire, EF. By 
inserting two resistances at a and D, and getting the balance upon EF, and 
then interchanging them, and getting the balance again, the difference of their 
resistances is obtained in terms of the resistance of the standard wire. For 
resistances inserted at A and D are equivalent to ungraduated prolongations 
of EF, and, therefore, when the balance has been obtained by adjusting 
the sliding contac, in order to maintain it after interchanging the conductor at 
A and D, the contact must be shifted a length of EF, the resistance of which is 
equal to the difference of the resistances at the two ends. When the resist- 
ance of one of the conductors A or D is negligible, that of the other is given by 
the shift of the sliding contact required to maintain the balance when this 
conductor is inserted; first at one of the end gaps and then at the other. 
Thus the wire, EF, becomes an adjustable standard of resistance, the indica- 
tions of which are unaffected by the resistance of the end connections. 
M. Béttger states that copper and brass may be coated with metallic zinc 
in the following way:—Finely divided zinc, in a non-metallic vessel, is 
covered with a concentrated solution of sal-ammoniac; this is heated to 
boiling, and the articles of copper or brass, properly cleansed, are introduced. 
A few minutes then suffice to produce a firm and brilliant coating. The 
requisite fineness of the zinc is produced by pouring the molten metal into a 
mortar, and triturating it until it solidifies. 
Not an unimportant feature in eletrical science is its application to military 
telegraphy. At a recent meeting of the Society of Telegraph Engineers, 
Capt. E. D. Malcolm, R.E., read an interesting paper, detailing the progress 
made in our military field telegraphs, as calculated to supply the communica- 
ting link between the outposts and the army. It appears that we have at pre- 
sent one troop of the Royal Engineer train devoted to this service, this troop 
being divided into three sections, each carrying twelve miles of wire in half- 
mile lengths. These pieces can be conveniently joined together by means of 
an ebonite jointer, which makes a practically water-tight joint in less than 
half a minute, and which, in the case of searching for faults, can be divided in 
a shorter time. The cable is a strand of seven No. 22 B. W.G. copper wires, 
insulated by Hooper’s compound. It is 3 in. thick and weighs 300 lbs. per 
mile. It is rolled on wooden drums, placed on waggons drawn by six horses. 
The poles are of wrought-iron tubing in two lengths, the butt ro feet long and 
1} inches in diameter, the top g feet long and 1 inch in diameter, fitted inside the 
lower half and fixed by a bayonet catch. The wire is carried and insulated in 
a wooden plug fitting into the top of the pole. The earth plates are of 
wrought-iron, 18 inches long, 44 inches wide, and } inch thick, and strong 
enough to be driven home in any soil; while they are accompanied by a 
six-gallon cask of water to insure moist earth. The office-van contains a pair 
of Morse recording instruments, fitted with Siemens’s polarised relay, and 
