moved slowly. If, however, the ring coil is held edgeways towards the magnet, and moved up (as in Fig. 26) towards the magnet pole, this kind of motion does not change the magnetic flux passing through the coil; because no lines. of magnetic flux are linked or unlinked with the circuit. of the coil, and hence under these conditions we find no current produced in the ring.*

The student should experiment with a coil and a bar magnet, and convince himself that there are certain motions of the ring which result in the production of electromotive force in the ring, and hence of an electric current; and, on the other hand, there are certain other motions of the ring in the neighbourhood of the magnet which do not result in the production of an induced electromotive force in the coil circuit. In the chapter specially devoted to Electromagnetic Induction we shall examine still more in detail this remarkable discovery, and show how it leads, in addition, to a practical way of measuring magnetic flux whether existing in the air or in an iron circuit completely closed.

* It need hardly be said that neither here nor elsewhere in this book do the diagrams represent the relative sizes or distances of the apparatus illus trated. In the above experiments (Figs. 25 and 26) it would be necessary in practice to place the galvanometer a sufficient distance from the magnet to prevent the needle being directly influenced by the magnet.

CHAPTER V.

THE MEASUREMENT OF ELECTRIC CURRENTS.

§ 1. The Magnetic Flux round an Electric Current. Whatever may be the form given to an electric circuit or wire conveying an electric current, there is always for the same circuit a definite field of magnetic flux round that circuit when traversed by a current; and it is necessary to examine the character of the flux in various cases. The reader should construct, therefore, in the first place, a coil of wire of about No. 20 S.W.G. (Standard Wire Gauge), having the form of a flat ring, preferably of nearly square cross-section.

In buying wire for experimental purposes in electrical work, the student should purchase double cotton covered copper wire of the necessary size. There are certain standard sizes of wire in use, the diameters of the bare wires being stated by numbers, according to the scale called the Standard Wire Gauge (S.W.G.), or the Birmingham Wire Gauge (B.W.G.), but the sizes for very accurate work are best stated in mils, one mil being the one-thousandth part of an inch. The following tables (p. 107) give data for the sizes most in use in electrical work.

The wire for electrical purposes is sold insulated with either cotton or silk. Double-cotton covered (d.c.c.) copper wire is good enough for most purposes unless high insulation is required, when the more expensive double-silk covered wire should be used. The wire should always be of that quality called high conductivity (H.C.) copper wire.

The double-cotton covering adds about 10 mils to the thickness of the wire, and the double-silk covering adds about 5 mils to the thickness. When a hank or bobbin of wire is purchased it should be first baked in the oven for an hour at a temperature rather above that of boiling water, and then boiled

TABLE OF COPPER WIRE GAUGES. Round Bare Wire.

Diameter of Wire in inches.

Standard

Gauge. Wire Gauge. Wire Gauge. make 1 lb. in.

S.W.G.

in white paraffin wax, to impregnate the cotton or silk, and prevent it from taking up moisture. To do this, procure a pound or two of pure white paraffin wax, and place it in a clean iron saucepan. Melt it carefully over the fire or a gas flame, and add a small lump or two of rosin. Then immerse the bobbin or coil of wire in this melted wax, and boil it until all bubbles cease to rise from the wire. Take care not to let the paraffin rise in temperature much above the temperature of boiling water, or so high as to smoke strongly. When the wire is impregnated pull out the bobbin or coil with a bit of bent wire and let it cool.

In making a flat ring coil of wire, the best way is to cut out a circular piece of wood, say of 4 inches in diameter, slightly conical, and an inch thick. Screw on to each side of this disk a rather larger disk of thin wood or thick cardboard. On this flat circular bobbin wind a coil of say fifty or one hundred turns of No. 20 cotton-covered copper wire, bringing the ends out through holes in the side disks. Then boil this coil, boards and all, as made, in paraffin wax, in a saucepan (which is best kept for the purpose), and when well boiled take it out and let it cool. When quite cold the side disks may be taken off and the coil of wire detached. The hard wax will cause the turns of wire to stick together, so that they retain the circular shape. The ring coil is then best preserved by winding it over with tape, which may be finally painted with shellac varnish. The inner and outer ends of the wire must be brought out through the tape. In making a coil for experimental purposes always count the number of turns of wire, and mark it on the coil.

Fix this flat coil between two pieces of thin board joined together but having nicks or cuts made in them to let the coil through (see Fig. 27). The coil must be so arranged that it is inclosed by the boards-half the coil being above and half below-and the centre of the coil on a level with the upper surface of the wood. The wood must then be neatly covered with white paper on its upper surface. Pass a rather strong current (four or five amperes at least) through a ring coil so arranged, and sprinkle fine steel filings all over the paper. Tap the board, and the filings will arrange themselves in a

series of curves, as shown in the figure. These filings delineate the lines of magnetic flux round the circular current. It will be seen that the flux goes through the aperture of the coil and then divides and returns round the outside, thus completing its circuit. If a small exploring magnetic needle or pocket compass is held in various positions near the coil, it will be found that the small magnetic needle places itself in every position along, or tangent to, the line of magnetic flux; and by following round a line of magnetic flux it will be seen to be linked with the wire of the electric circuit or axis of the electric current. It is therefore commonly said

Fig. 27. The dotted lines show the Direction of the Magnetic
Flux round a Circular Conductor conveying a Current; the
lines being taken on a horizontal plane through the centre.

that a current flowing in a circuit generates lines of magnetic flux round the circuit which are linked with that circuit.

It is, however, better to think of the wire as simply forming the circular axis round which there is a reentrant magnetic flux, or circuit of magnetic flux. It will also be seen that in no place except just at the very centre of the coil are the lines of magnetic flux parallel and straight.

The reader should, in the next place, explore the form of the magnetic flux round a long coil or solenoid,