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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



a description of which, without the aid of draw- ' 

 ings, would be too technical for our readers. A 

 submarine cable is itself, however, a particular 

 form of such a condenser. The copper wire is 

 one of the opposed conductors, the sheathing, 

 earth, and sea-water, form the other, and these 

 are separated from each other by the insulating 

 coating of gutta-percha. It is a curious fact that 

 when a charge of electricity is communicated to 

 the copper wire of a cable, it induces a charge of 

 an opposite kind in the earth outside. This in- 

 ductive property of an insulated wire contiguous 

 to the earth has an important bearing on practi- 

 cal telegraphy ; for, inasmuch as the communi- 

 cated charge and the induced charge attract each 

 other, the former travels less swiftly along the 

 wire ; it is held back, as it were, by the retarding 

 influence of the earth's induced charge; or, in 

 other words, has a tendency to ooze out of the 

 cable instead of traveling uninterruptedly to the 

 other end. It is of consequence, therefore, to 

 ascertain the inductive capacity of a cable ; as 

 the less it is, the greater will the speed of sig- 

 naling be. 



The resistances and capacity of a cable are 

 usually tested, according to the standards of re- 

 sistance and capacity — that is, with the ohm, 

 meg-ohm, and micro-farad — by measuring the 

 strength of an electric current passing through 

 the cable, by means of an instrument called the 

 galvanometer, or current-measurer. Its principle 

 depends upon the fact, discovered by Oersted, 

 the famous Copenhagen philosopher, that when a 

 current is sent along a wire in the neighborhood 

 of a freely-suspended magnetic needle, the needle 

 will be deflected into a new position, and this po- 

 sition will be to right or to left according as the 

 current of one kind or the other is sent through 

 the wire. Moreover, the amount of deflection 

 will be directly proportional to the strength of the 

 current. This great discovery, which gave an 

 incalculably great impetus to the progress of the 

 telegraph, is the theoretical basis of the galvanom- 

 eter. One form of this instrument, used to test 

 submarine cables, is called the " reflecting gal- 

 vanometer," and is the invention of Sir William 

 Thomson. The wire through which the current 

 to be measured is made to pass, consists of a 

 great many turns of silk-covered or insulated 

 copper of a very fine gauge, forming a hollow 

 coil, in the heart of which a very diminutive mag- 

 netic needle is suspended by a gossamer-like fila- 

 ment of floss-silk. This magnet (or magnets, for 

 there are generally more than one) carries a tiny 

 circular mirror, the whole arrangement of mag- 



nets and mirror being no longer than (^). A 

 beam of light is thrown from a lamp in front on to 

 the mirror, and reflected back again on to a grad- 

 uated pasteboard scale. When the current to be 

 measured is sent throughout the coiled wire sur- 

 rounding the magnets, they are turned horizon- 

 tally on their former position, and the mirror is 

 inclined round with them, so that the reflected 

 beam of light is moved along the scale, the dis- 

 tance to which it is moved being a measure of 

 the current strength. 



Now, when the current from a given battery 

 or source of electricity is made to pass through 

 wires of different resistances, the strength of the 

 current which will pass through these wires can 

 be made a measure of their resistances ; and there- 

 fore, when the current from a particular bat- 

 tery is sent through the conductor of the cable 

 or to test the insulator, and in each case meas- 

 ured by the galvanometer, and compared with the 

 current from the same source which will flow 

 through the units of comparison, the copper re- 

 sistance and insulation resistance can be obtained. 



In a somewhat similar way the capacity — the 

 amount of electricity which a cable will take — is 

 compared with the capacity of a standard con- 

 denser or measure of capacity. The opposite 

 plates or sheets of the condenser are charged by 

 a particular battery; and, as these charges are 

 eager to flow into each other and unite, but are 

 held apart by the insulator, they may be allowed 

 to do so through a wire or other conductor. The 

 discharge of the opposite electricities into one 

 another sets up a short, powerful current in this 

 wire, and its strength is proportional to the quan- 

 tity of electricity discharged ; that is, to the ca- 

 pacity of the condenser. If the coil of the gal- 

 vanometer be substituted for this discharging-wire 

 the strength of this discharge will be measured 

 by the deflection of the gleam of light on the 

 scale. By charging alternately, therefore, the 

 condenser and the cable from the same battery, 

 and observing their respective discharges by 

 means of the galvanometer, the capacities of the 

 cable and condenser are compared. 



The speed of signaling through a submarine 

 cable, that is to say, the number of words per 

 minute that can be transmitted through it, varies 

 with the resistance of its conductor, its inductive 

 capacity, and its length ; and it is by a considera- 

 tion of these properties, together with weight and 

 cost of material, that its form and dimensions are 

 designed ; and on this interesting subject we may 

 have a few words to say in a future paper. — 

 Chambers' 's Journal. 



