is directly proportional to the angular deflection up to 45° or 50°. (See page 147 for plan used when coil moves.)

Fig. 65.-Mather and Walmsley's Proportional Galvanometer.

Even although the controlling magnet of a galvanometer be rather near the needle, the controlling field may be regarded as an approximately uniform one if the deflections of the needle be all very small. Similarly for very small deflections the controlling field may be regarded as approximately uniform whatever be the shape and size of the coil or of the needle. If then, in addition, the controlling magnet be so placed that when no current is passing the needle makes about the same small angle with the plane of the coil on one side of it that it makes with that plane on the other side for the greatest deflection employed, the distribution of the forces will

Fig. 06.

be as in Fig. 66, where N P represents the magnitude and direction of the controlling force, P R, the magnitude and direction of the deflecting force for current 1, P R2 for current 2, P Rg for current 3, &c., P R 2 being twice PR1, P Rg three times P R1, &c.

Therefore the angular deflections of the needle for currents 1, 2, 3, &c., are PNR1, PN R2, P N R3, &c., and, as these angles are all very small, and the base lines are proportional to the currents, it follows that the angular deflections are also proportional to the Indeed for very small deflections this result will be nearly true, whether the angle N P X is a little less than, or a little more than, or exactly equal to a right angle; that is, whatever be the angle the needle makes with the plane of the coil provided that this angle is small.

currents.

34. Galvanometers of Invariable Sensibility.-Now that measuring currents in amperes has acquired the same sort of practical importance as weighing coals in tons or finding the number of cubic feet of gas passing through a pipe, it is necessary to have galvanometers which are portable, and whose indications are not affected by moving the galvanometer from one place to another, or by placing it near an iron pipe, a fire-place, or even near the powerful electromagnets of the dynamo machines which are employed for the mechanical production of electric currents.

An instrument of this type must be “direct-reading”; that is, the deflection of the pointer must indicate at once the current in amperes, for in commercial work there is no time to refer to a table of values, not to mention the risk that would be introduced by a table of values belonging to some other instrument being used by mistake.

Such instruments, by means of which the current can be read off at once in amperes without any calculation or reference to any calibration curve, are called " ammeters," and during the last twelve years so much

attention has been given to the design and construction of this class of electrical meter that it is now possible to measure a current with as much accuracy as a leg of mutton can be weighed in a pair of scales, or with a spring-balance, and with even greater facility.

The controlling force must necessarily be exerted in such a way that it is the same wherever the ammeter is placed, indeed, many ammeters are so constructed that the controlling force is not changed by laying the instrument on its side, or in any other position, so that a current can be read off equally well whether the ammeter is lying on a table, hung up on a wall, held in the hand, or used on board a ship rolling in a heavy sea.

There are three distinct ways in which the controlling force is exerted in ammeters.

(1) By means of a powerful permanent magnet placed inside the instrument and rigidly fixed to it.

(2) By means of a spring.

(3) By means of a weight.

The first two methods have the advantage that with their use the moving part of the ammeter can be balanced like a wheel in a watch, so that the instrument can be made to read correctly in any position; the former of these two has also the further advantage that as the control exerted by a powerful magnet close to the needle is very large, outside magnetic disturbances have little effect. But while a magnet or a spring can be made constant enough in their action for many practical purposes, their variation with time is of course greater than that of a weight, hence the third method of control is the one adopted when accuracy is of more importance than portability.

In the earlier editions of this book several ammeters were described, and their advantages and disadvantages compared. But the methods of constructing the coils and needles, and the various devices that are now adopted in applying the controlling force in one or other of the

three ways just referred to have become so numerous, that anything like a complete description of all the types of ammeters now in use and an examination of their relative advantages would alone fill a good-sized book.

Ammeters, besides differing in the methods used for exerting the controlling force, also differ in design, depending on whether the instrument is intended to measure currents of very different values, or only currents all of about the same value. In the former case the design should be such that the scale is equally or nearly equally divided, so that there is about the same distance between any adjacent pair of division marks, while in the latter the scale should be very "open"; that is, the division marks should be widely separated at the one part of the scale which is in constant use, and crowded together at those parts which correspond with currents which rarely have to be measured. Instruments with this latter type of scale are especially employed when an ammeter is used to indirectly measure "electric pressure" (see § 49, page 179).

In § 2 we saw that when a conductor conveying a current is placed near a magnet there is a force exerted between the conductor and the magnet, tending to make them move relatively to one another.

The force acts in such a direction that a wire carrying a current tends to move perpendicular to itself and perpendicular to the lines of force. It is only when the wire lies along the lines of force that the action between it and the magnet is nought, however strong be the current and however powerful the magnetic field. With any other position of the wire relatively to the direction of the magnetic field there is some force, and this force has its greatest value for a given length of conductor carrying a given current, and placed in a field of given strength, when the conductor is perpendicular to the lines of force.

By employing a very powerful magnet the force. exerted on a wire, even when conveying a feeble current,

F

can be made considerable, and this action has been employed by Maxwell, Lord Kelvin, Deprez, d'Arsonval, and others, to obtain galvanometers which are not only very sensitive, but the indications of which are very little affected by extraneous magnetic disturbance.

Fig. 67.-Framework of Ayrton and Perry's Permanent Magnet Ammeter.

EARLY FORMS OF AMMETERS.

35. Permanent Magnet Ammeter. The earliest ammeter, having an equally divided scale so that the deflection in degrees was directly proportional to the current, was the "permanent magnet ammeter" devised by Professor Perry and the author in 1880. The coil