Page:The Granite Monthly Volume 1.djvu/117

 AN OCEAN CABLE.

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��its subsequent immersion beneath the waters of the ocean.

In order to measure the strength of the current, it is plain that some fixed standard of measurement must be adopt- ed; hence the electricians use "Siemen's Unit" and the "Ohm," which are con- vertible terms, and are called the "units of resistance," in the same manner that an inch, a foot, a mile, are standard meas- urements of length. For convenience, a very small wire of pure copper, weigh- ing but a pound to the nautical mile, is assumed as a standard, and is found to offer to the passage of the electric cur- rent a resistance of 1091.22 ohms, when the wire is at a temperature of 32 de- grees, or the freezing point of Fahren- heit. It must be borne in mind right here that the temperature of the wire has a marked effect upon the transmission of the current, and adds another impor- tant factor to the complication of the problem. The resistance increases direct- ly with the length of a cable, and de- creases inversely with its weight ; that is to say, a large wire will offer less re- sistance to the current than a small one, because it contains a greater amount of conducting surface.

A very curious thing about the in- creased resistance, coincident with the rise in temperature, is that the resistance increases in exactly the same manner as a sum of money put out at compound interest. The resistance compounds it- self with every degree of increasing heat. For instance, it has been found that each degree increases the resistance eight thou- sand nine hundred and fifty ten-millionths of an ohm (.0008950). Now the resist- ance at 32 degrees being 1091.22 ohms, the resistance at 33 degrees will be 1091.22 plus .0008950=1091.2208950, and the resistance for 34 degrees will be the latter sum plus the .0008950, and so on. This law enables the electrician to con- struct tables for the reduction of read- ings of resistance to the standard of 75 degrees Fahrenheit, and for a calculation for the reading at any temperature, from a test made at any other. Knowing these and other minor facts, whenever a cable ceases to work, the electrician has his tables of resistance and other mathe-

��matical computations at hand to aid him in his work, just as the bank accountant consults his interest tables, or the sur- veyor his table of logarithms.

The process of testing is materially shortened by the use of the table of log- arithms, as it enables the electrician to avoid the long and tedious processes of multiplication, division, and extraction of roots. The exact resistance of a per- fect cable of a given length being known, the electrician constructs a perfect arti- ficial one of any required length, and compares the defective one with it. This statement may seem improbable, and the natural inference w r ould be that it would not be very convenient to have anywhere from 500 to 3000 miles of cable piled up in an office. But it is quite convenient — in fact, absolutely indispensable— and a thousand or two miles of ocean cable may be piled up in a box on a common table. This seeming paradox is ex- plained by the fact already alluded to, that the resistance to the electric current decreases inversely with the weight of the wire. Consequently a coil of very small wire will represent the same amount of resistance as a much greater length of larger wire — exactly upon the same principle that an architect can rep- resent the immense centennial buildings upon a card, or the artist a vast extent of landscape upon a small strip of can- vas. For the purpose of testing, "resist- ing coils" are prepared, consisting of wire drawn out exceedingly fine and wound upon reels like a fish-line. They are placed in different sections of a box made for the purpose, and thoroughly tested, and graduated to represent so many hun- dred or thousand units of resistance (ohms). Metallic pins, connecting with the battery, are so adjusted as to connect or disconnect the coil with the battery, at the pleasure of the operator. Thus the electrician can construct in a minute, right before him, a perfect cable of any required length.

His real cable, which lies at the bottom of the ocean, having ceased to work, he has only to compare it with the artificial one before him to find out where the trouble is. He hitches up the wire of the defunct cable to the reflecting galvan-

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