The three units--the volt, ampere and ohm--are so related that a pressure of one volt acting upon a circuit with a resistance of one ohm will produce a current of one ampere.
A current can do work; when it lights or heats your room or drives a tramcar it is doing work; and the rate at which a current does work is found by multiplying together the number of volts and the number of amperes. The result is in still another unit, the watt. And 1000 watts is a kilowatt. Finally, to crown the whole story, a kilowatt for one hour is a Board of Trade unit.
So for every unit which you pay for in the quarterly bill you have had a current equal to 1000 watts for an hour. To give a concrete example, if the pressure of your supply is 200 volts, and you take a current of five amperes for an hour, you will have consumed one B.T.U.
Perhaps it will give added clearness to this explanation to tabulate the terms as follow:-- Volt = The unit of pressure, analogous to "pounds per square inch" in the case of water. Coulomb = The measure of quantity, analogous to the gallon.
Ampere = The measure of the "strength" of a current, meaning one coulomb per second. [25]
Watt = The unit denoting the power for work of any current. It is the result of multiplying together volts and amperes. Kilowatt = 1000 watts.
Board of Trade Unit = A current of one kilowatt flowing for one hour.
In practice the measurements are generally made by means of the connection between electricity and magnetism. A current of
electricity is a magnet. Whenever a current is flowing it is surrounded by a region in which magnetism can be felt. This region is
called the magnetic field, and the strength of the field varies with the strength that is the number of amperes in the current. If a wire carrying a current be wound up into a coil it is evident that the magnetic field will be more intense than if the wire be straight, for it will be concentrated into a smaller area. Iron, with its peculiar magnetic properties, if placed in a magnetic field seems to draw the magnetic forces towards itself, and consequently, if the wire be wound round a core of iron, the magnetism due to the current will
be largely concentrated at the ends of the core. But the main principle remains--in any given magnet the magnetic power exhibited
will be in proportion to the current flowing.
The switchboard at a generating station is always supplied with instruments called ammeters, an abbreviation of amperemeters, for the purpose of measuring the current passing out from the dynamos. Each of these consists of a coil of wire through which the current passes. In some there is a piece of iron near by, which 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-mag- net, or coil, and the permanent magnet being such that they approach each other or recede from each other[26] 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 be-ing 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
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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 en-ergy 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.
[27]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 great-
est 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[28] 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