cient to injure the insulation and allow the electricity to escape into the water, which is a natural conductor of it. It would be precisely the same as if a large hole were bored in the under side of a lead pipe conveying water to a tank.
Supposing, however, that the fracture is complete, or at least serious enough to destroy the usual communication between the operators at the two ends. It is obvious that the cable must be grappled up and repaired, or it is entirely worthless. A single inch of defective wire renders the whole of no account. Before it can be used again a ship is sent out, furnished with the necessary grappling apparatus, to the vicinity where the trouble exists. The "fault" is cut out a new splice made, and the cable is dropped again upon the bottom of the ocean.
One of the principal instruments used in "testing" is known as Thomson's reflecting galvanometer, and is the invention of Sir William Thomson of Glasgow University, Scotland. It is a small instrument, of elegant workmanship and extreme delicacy, consisting of a very small magnetic needle about three-eighths of an inch long, fixed to the back of a small circular mirror, whose diameter is about equal to the length of the magnet. In this respect it is similar to the mirror used in reading messages. The mirror is sometimes a plano-convex lens of about six feet focus, and is suspended from the circumference by a single cocoon fiber without torsion—the magnetic needle being at right angles with the fiber. The cocoon fiber is silk in its raw state, infinitely finer than the finest manufactured thread, and of course susceptible to the slightest movement in any direction. The mirror is placed in the axle of a coil of wire, some four or five inches across, which completely surrounds it, so that the needle is always under the influence of the coil at whatever angle it is deflected to. A beam of light from a lamp placed behind a screen about three feet distant from the coil falls on the little mirror, the bottom of which is slightly in advance of the top, and is reflected back on to a graduated scale placed just above the point where the beam of light emerges from the lamp. The screen is, as we have before said, straight, and is graduated to 360 divisions on either side of the zero point. This scale being placed about three feet distant from the mirror, it is obvious that a very small angular movemeut of the little mirror will cause the spot of light reflected on the scale to move a considerable distance across it.
A very good illustration of how this operation may be accomplished, is to take a common looking-glass, hold it in front of you, place a lighted lamp or candle in front, and notice the spot of light that will be reflected upon the wall of the room behind the lamp. Turn the glass from side to side, and of course the spot of light upon the wall moves in a corresponding manner. A similar phenomenon may sometimes be observed in the school-room, when some roguish youth directs the light of his little pocket mirror to the opposite side of the house and causes the concentrated rays of the sun to illuminate the optics of some juvenile sweetheart.
The needle of the galvanometer being very small, and being placed in the center of the coil of wire previously alluded to, every current of electricity deflects it, and deflects it to a degree directly proportional to the strength of the current. This being a known fact, the next step is to know how much electricity is sent out from a given battery, and where it goes to. Also to know if it meets with any resistance on its passage, and if so, how much, and where. The solution of these problems covers the whole ground of "testing." First of all, then, the materials of which the cable is composed must be known, and the exact amount of resistance that the metal offers to the passage of the electrical current. No two metals have the same power of conductivity, and a wire composed of pure copper will offer less resistance than one composed of the baser metals. The weight of the central or conducting wire of the direct cable is between 300 and 400 pounds to the mile, and is made of pure copper. The exact amount of resistance that it offers is known by the thousands of tests made during its manufacture and
