A'

meter are then taken, the resistance being gradually decreased. When the current has reached its maximum value the resistance is again gradually increased and the current reduced to zero; if the results be plotted it will be found that the descending curve is much less steep than the ascending, and when the current is zero there will be a considerable amount of residual magnetism left. The battery

commutator is then reversed, and the resistance again diminished until the current reaches a maximum negative value. It will be found that during this process the magnetisation does not at first alter much, but that after the current has attained a not very large negative value there is a sudden large change in the magnetisation from a considerable positive amount to an equally large negative value. After this, as the current increases the magnetisation in

A

creases, but more gradually. When the current has reached its maximum negative value it is again decreased by increasing the resistance, and afterwards, passing through zero, reversed and increased again up to the same positive maximum as before.

If the magnetisation curve for this process be drawn, it will be a closed curve, resembling in form that given in figure xlii.

Again, it has been shewn that the area measured on a proper screen of the closed cycle is the total energy required to carry unit volume of the iron through the magnetic changes. This energy is dissipated as heat.

Moreover, Prof. Ewing has shewn that whenever iron is taken through any cyclic process of magnetising force, the magnetisation changes, but in such a way as always to lag behind the magnetising force; there is a tendency for the existing state of magnetisation to persist. To this tendency he has given the name hysteresis, and it is in consequence of this hysteresis that energy is required to produce a cycle of magnetic changes.

Experiments.

(1) Determine the magnetic moment of the given pieces of soft iron under a given magnetic force.

(2) Find the susceptibility and permeability of soft iron for various values of the magnetising force, and determine also the residual magnetisation when the force is suddenly removed.

(3) Draw the hysteresis curve for the given specimen of soft iron, and calculate the energy dissipated as heat in carrying it round a complete cycle.

Enter in parallel columns the values of H, I, «, B, μ, and draw the curve.

1

For a discussion of the properties of this curve, and the variations in its form for various specimens of iron, see Ewing, Magnetic Induction in Iron, &c., chaps. iv. and v., from which much of the above

is taken.

CHAPTER XVIII.

ELECTRICITY-DEFINITIONS AND EXPLANATIONS OF
ELECTRICAL TERMS.

IN the last chapter we explained various terms relating to magnetism. Just as in the neighbourhood of a magnet we have a field of magnetic force, so, too, in the neighbourhood of an electrified body there is a field of electric force. We proceed to consider certain facts, and to explain some of the terms connected with the theory of electricity, a clear comprehension of which will be necessary in order to understand rightly the experiments which follow.

Most bodies can by friction, chemical action, or by various other means, be made to exert forces on other bodies which have been similarly treated. The phenomena in question are classed together as electrical, and the bodies are said to have been electrified. By experiments with Faraday's icepail among others (vide Maxwell's 'Elementary Electricity,' p. 16, &c.), it has been shewn that these effects can be accounted for by supposing the bodies to be charged with certain quantities of one of two opposite kinds of electricity, called respectively positive and negative, and such that equal quantities of positive and negative electricity completely. annihilate each other.

An electrified body exerts force on other electrified bodies in its neighbourhood-in other words, produces at field of electrical force-and the force at any point depends on the position of the point, on the form and dimensions of the electrified body, and on the quantity of electricity on the body. By doubling the charge we can double the force. We are thus led to look upon electricity as a quantity which can be measured in terms of a unit of its own kind, and we may speak of the quantity of electricity on a body, in somewhat the same way as we use the term quantity of magnetism for the strength of a magnetic pole. The magnetic forces

produced by a magnetic pole are due to a quantity of magnetism concentrated at the pole. The electrical forces produced by an electrified body are due to a quantity of electricity distributed over the body. By supposing the body to become very small while the quantity of electricity on it still remains finite, we may form the idea of an electrified point or a point charged with a given quantity of electricity.

With regard to the transmission of electrical properties bodies may be divided into two classes, called respectively conductors and non-conductors. To the latter the name 'dielectric' is also applied.

DEFINITIONS OF CONDUCTORS AND NON-CONDUCTORS.If a quantity of electricity be communicated to a conductor or conducting body at one point, it distributes itself according to certain laws over the body; if, on the other hand, it be communicated to a non-conductor, it remains concentrated at the point where it was first placed. Quantities of electricity pass freely through the substance of a conductor; they cannot do so through a non-conductor.

Quantities of electricity are of two kinds, having opposite properties, and are called positive and negative respectively. Two bodies each charged with the same kind of electricity repel each other; two bodies charged with opposite kinds attract each other. To move an electrified body in the field of force due to an electrified system, against the forces of the system requires work to be done, depending partly on the forces of the system and partly on the quantity of electricity on the body moved. We shall see shortly how best to define the unit in terms of which to measure that quantity.-Moreover, owing to the action between the electrified body and the rest of the system, alterations will generally be produced in the forces in consequence of the motion.

DEFINITION OF RESULTANT ELECTRICAL FORCE.-The resultant electrical force at a point is the force which would be exerted on a very small body charged with unit quantity of positive electricity placed at the point, it being supposed

that the presence of the body does not disturb the electrification of the rest of the system.

Hence if R be the resultant electrical force at a point, and the number of units of electricity at that point, the force acting on the body thus charged is Re

If the body so charged be moved by the forces acting on it, work is done.

DEFINITION OF ELECTROMOTIVE FORCE. The work done in moving a unit quantity of positive electricity from one point to another is called the electromotive force between those points.

Hence, if the electromotive force (denoted by the symbols E.M.F.), between two points be E, the work done in moving a quantity e of positive electricity from the one point to the other is E e. Electromotive force is sometimes defined as the force which tends to move electricity; the definition is misleading. The name itself is perhaps ambiguous, for the electromotive force between two points is not force, but work done in moving a unit of positive electricity; it, therefore, has the dimensions of work divided by electrical. quantity (see p. 20). The term electromotive force at a point, however, is sometimes used as equivalent to the resultant electrical force. We shall avoid the term.

Suppose that a single body charged with positive electricity is being considered, then it is found that the force which this body exerts on any electrified body decreases very rapidly as the distance between the two bodies is increased, becoming practically insensible when the distance is considerable. We may define as the field of action of an electrified system of bodies that portion of space throughout which the electrical force which arises from the action of those bodies has a sensible value. If a quantity of positive electricity be moved from any point of the field to its boundary by the action of the electrical forces, work is done.

DEFINITION OF ELECTRICAL POTENTIAL.-The electrical potential at a point is the work which would be done by the