Marvels of Scientific Invention - Thomas W. Corbin

Скачать книгу

is attracted more or less as the current varies, the iron being pulled back by a spring and its movement against the tension of the spring being indicated by a pointer on a dial.

      In others the coil itself is free to swing in the neighbourhood of a powerful steel magnet, the interaction between the electro-magnet, or coil, and the permanent magnet being such that they approach each other or recede from each other as the current varies. A pointer on a dial records the movements as before.

      In yet another kind the permanent magnet gives way to a second coil, the current passing through both in succession, the result being very much the same, the two coils attracting each other more or less according to the current.

      Another kind of ammeter known as a thermo-ammeter works on quite a different principle. It consists of a piece of fine platinum wire which is arranged as a "shunt"—that is to say, a certain small but definite proportion of the current to be measured passes through it. Now, being fine, the current has considerable difficulty in forcing its way through this wire and the energy so expended becomes turned into heat in the wire. It is indeed a mild form of what we see in the filament of an incandescent lamp, where the energy expended in forcing the current through makes the filament white-hot. The same principle is at work when we rub out a pencil mark with india-rubber, whereby the rubber becomes heated, as most of us have observed. The wire, then, is heated by the current passing through it, and accordingly expands, the amount of expansion forming an indication of the current passing. The elongation of the wire is made to turn a pointer.

      A simple modification makes any of these instruments into a voltmeter. This instrument is intended to measure the force or pressure in the current as it leaves the dynamo.

      A short branch circuit is constructed, leading from the positive wire near the dynamo to the negative wire, or to the earth, where the pressure is zero. In this circuit is placed the instrument, together with a coil made of a very long length of fine wire so that it has a very great resistance. Very little current will flow through the branch circuit because of the high resistance of the coil, but what there is will be in exact proportion to the pressure. The voltmeter is therefore the same as the ammeter, except that its dial is marked for volts instead of for amperes, and it has to be provided with the resistance coil.

      Instruments of the ammeter type can also be used as ohmmeters. In this case what is wanted is to test the resistance of a circuit, and it is done by applying a battery, the voltage of which is known, and seeing how much current flows.

      All the voltmeters and ohmmeters mentioned owe their method of working to what is known as Ohm's law. One of the greatest steps in the development of electrical science was taken when Dr. Ohm put forward the law which he had discovered whereby pressure, current and resistance are related. The reader will probably have noticed from what has already been said about the units of measurement—the volt, the ampere and the ohm—that the current varies directly as the pressure and inversely as the resistance. That is the famous and important "Ohm's law" and anyone who has once grasped that has gone a long way towards understanding many of the principal phenomena of electric currents.

      But the instruments so far referred to are of the big, clumsy type, suitable for measuring large currents and great pressures. They are like the great railway weigh-bridges, which weigh a whole truck-load at a time and are good enough if they are true to a quarter of a hundredweight. The instruments about to be described are more comparable with the delicate balance of the chemist, which can detect the added weight when a pencil mark is made upon a piece of paper. Indeed beside them such a balance is quite crude and clumsy. They may be said to be the most delicate measuring instruments in existence.

      We will commence with the galvanometer. The simplest form of this is a needle like that of a mariner's compass very delicately suspended by a thin fibre in the neighbourhood of a coil of wire. The magnetic field produced by the current flowing in the wire tends to turn the needle, which movement is resisted by its natural tendency to point north and south. Thus the current only turns the needle a certain distance, which distance will be in proportion to its strength. The deflection of the needle, therefore, gives us a measure of the strength of the current.

      But such an instrument is not delicate enough for the most refined experiments, and the improved form generally used is due to that prince of inventors, the late Lord Kelvin. He originally devised it, it is interesting to note, not for laboratory experiments, but for practical use as a telegraph instrument in connection with the early Atlantic cables.

      Before describing it, it may sharpen the reader's interest to mention a wonderful experiment which was made by Varley, the famous electrician, on the first successful Atlantic cable. He formed a minute battery of a brass gun-cap, with a scrap of zinc and a single drop of acidulated water. This he connected up to the cable. Probably there is not one reader of this book but would have thought, if he had been present, that the man was mad. What conceivable good could come of connecting such a feeble source of electrical pressure to the two thousand miles of wire spanning the great ocean; the very idea seems fantastic in the extreme. Yet that tiny battery was able to make its power felt even over that great distance, for the Thomson Mirror Galvanometer was there to detect it. Two thousand miles away, the galvanometer felt and was operated by the force generated in a battery about the size of one of the capital letters on this page.

      This wonderful instrument consisted of a magnet made of a small fragment of watch-spring, suspended in a horizontal position by means of a thread of fine silk, close to a coil of fine wire. When current flowed through the coil the magnetic field caused the watch-spring magnet to swing round, but when the current ceased the untwisting of the silk brought it back to its original position again.

      So far it seems to differ very little from the ordinary galvanometer previously mentioned, but the stroke of genius was in the method of reading it. With a small current the movement of the magnet was too small to be observed by the unaided eye, so it was attached to a minute mirror made of one of those little circles of glass used for covering microscope slides, silvered on the back. The magnet was cemented to the back of this, yet both were so small that together their weight was supported by a single thread of cocoon silk. Light from a lamp was made to fall upon this mirror, thereby throwing a spot of light upon a distant screen. Thus the slightest movement of the magnet was magnified into a considerable movement of the spot of light. The beam of light from the mirror to the screen became, in fact, a long lever or pointer, without weight and without friction.

      The task of watching the rocking to and fro of the spot of light was found to be too nerve-racking for the telegraph operators, and so Lord Kelvin improved upon his galvanometer in two ways. He first of all managed to give it greater turning-power, so that, actuated by the same current, the new instrument would work much more strongly than the older one. Then he utilised this added power to move a pen whereby the signals were recorded automatically upon a piece of paper. The new instrument is known as the Siphon Recorder.

      The added power was obtained by turning the instrument inside out, as it were, making the coil the moving part and the permanent magnet the fixed part. This enabled him to employ a very powerful permanent magnet in place of the minute one made of watch-spring. The interaction of two magnets is the result of their combined strength, and that of the coil being limited by the strength of the minute current the only way to increase the combined power of the two was to substitute a large powerful magnet for the small magnetised watch-spring. This large magnet would, of course, have been too heavy to swing easily and therefore the positions had to be reversed.

      But there is a more wonderful story still to come, of an instrument which can detect a millionth of a micro-ampere, or one millionth of a millionth of an ampere. It is not generally known that we are all possessors of an electric generator in the form of the human heart, but it is so, and Professor Einthoven, of Leyden, wishing to investigate these currents

Скачать книгу