until all are at a safe distance, so that a premature explosion is impossible.
In many of the cases described, the shock takes place almost entirely within the earth and there is very little debris thrown about.
Indeed the only danger which is to be feared with these operations is about on a par with that which every farm hand runs from the kick of a horse. Any careful, trustworthy man could be quite safely taught to do this work, and with the assistance of a labourer he could do all that is necessary. Given a fair amount of intelligence, too, he would take but little teaching. Altogether there is no doubt that the use of explosives is going to have a marked effect upon farming operations in the near future.
CHAPTER II
MEASURING ELECTRICITY
There are many people whose acquaintance with electricity consists mainly in paying the electric light bill. To such the instruments whereby electricity is measured will make a specially interesting appeal.
Current is sold in Great Britain at so much per Board of Trade Unit. To state what that is needs a preliminary explanation of the other units employed in connection with electric currents.
The public electricity supply in any district is announced to be so many volts, it may be 100, 200 or perhaps 230, but whatever it be, it is always so many "volts." Then the electrician speaks lightly of numbers of "amperes," he may even talk of the number of "watts" used by the lamps, while occasionally the word "ohm" will leak out. Among these terms the general reader is apt to become completely fog-bound. But really they are quite simple if once understood, and, as we shall see in a moment, there are some very remarkable instruments for measuring them, some of which exhibit a delicacy truly astonishing.
It is well at the outset to try and divest ourselves of the idea that there is anything mysterious or occult about electricity. It is quite true that there are many things about it very little understood even by the most learned, but for ordinary practical purposes it may be regarded as a fluid, which flows along a wire just as water flows along a pipe. The wire is, electrically speaking, a "hole" through the air or other non-conducting substance with which it is surrounded. A water-pipe being a hole through a bar of iron, so the copper core of an electrical wire is, so far as the current is concerned, but a hole through the centre of a tube of silk, cotton, rubber or whatever it be. Electricity can flow through certain solids just as water can flow through empty space.
Water will not flow through a pipe unless a pressure be applied to it somewhere. In a pipe the ends of which are at the same level water will lie inert and motionless. Lower one end, however, and the pressure produced by gravity—in other words, the weight of the water—will cause it to move. In like manner pressure produced by the action of a pump will make water flow. On the other hand, when it moves it encounters resistance, through the water rubbing against the walls of the pipe.
Similarly, an electrical pressure is necessary before a current of electricity will flow. And every conductor offers more or less resistance to the flow of current, thus opposing the action of the pressure. Before current will flow through your domestic glow-lamps and cause them to give light there must be a pressure at work, and that pressure is described as so many volts.
A battery is really a little automatic electrical pump for producing an electrical pressure. And the volt, which is a legal measure, just as much as a pound or a yard, is a certain fraction of the pressure produced by a certain battery known as Clark's Cell. It is not necessary here to say exactly what that fraction is, but it will give a general idea to state that the ordinary Leclanche or dry cell, such as is used for electric bells, produces a pressure of about one and a half volts.
Thus we see the volt is the electrical counterpart of the term "pound per square inch" which is used in the case of water pressure.
A flow of water is measured in gallons per minute. An electrical current is measured in coulombs per second. Thus the coulomb is the electrical counterpart of the gallon. But in this particular we differ slightly in our methods of talking of water and electricity. Gallons per minute or per hour is the invariable term in the former case, but in the latter we do not speak of coulombs per second, although that is what we mean, for we have a special name for one coulomb per second, and that same is ampere. One ampere is one coulomb per second, two amperes are two coulombs per second, and so on.
There is no recognised term to denote the resistance which a water-pipe offers to the passage of water through it, but in the similar case with electricity there is a term specially invented for the purpose, the ohm. Legally it is the resistance of a column of mercury of a certain size and weight. A rough idea of it is given by the fact that a copper wire a sixteenth of an inch thick and 400 feet long has a resistance of about one ohm.
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.
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,