of the machine recommences, either in the same way as before or in the opposition, as the case may be.

If the jars are connected with the conductors, the flow, instead of passing continuously across the gap ab, charges the jars until the potential reaches the value corresponding to the striking distance; a spark passes and the same succession of phenomena is repeated. When the jars are used, if the distance of the two poles is too great for the spark to pass, it sometimes happens that the action stops, then starts again, the signs of the poles being reversed, and the same series of alternations are reproduced. Each jar is periodically charged and discharged by the corresponding comb. This is specially liable to occur with batteries of large capacity.

Holtz's machine affords a means of making a curious experiment on reversibility. If the two combs of a machine in the ordinary state are connected with the poles of a second one, from which the driving band has been removed so as to allow the plate to turn freely, and if the first machine is then set in action, and a slight impulse is given to the plate of the second machine, it continues to rotate; the electricity thus transmits the motion of the first machine to the second.

92. Diametral Conductor.-Various attempts have been made to prevent the machine from ceasing to act or reversing its poles. A useful contrivance for this

purpose is the diametral conductor (Fig. 87). Each armature occupies almost the whole of a quadrant, and an insulated conductor, A'B', with combs at the ends, can be placed between them along any diameter. So long as the two poles are in contact, or when they are separated, so long as the machine continues in normal action, the

B'A

FIG. 87.

diametral conductor has no effect, the conditions of its equilibrium between the two armatures being those which have been established by the principal conductor. But if the poles are separated so far that the principal conductor ceases to act, the diametral conductor replaces it, and the machine continues to work.

93. Voss's Machine.-Holtz's machine has been modified in many ways. Voss's machine (Fig. 88), which is now widely used,

is a machine with a diametral conductor, and has some of the features of the Holtz machine and of the replenisher.

The movable plate is provided with metal studs, which act like the carriers of the replenisher. The inductors are represented by two strips of tinfoil fixed to the centre of the paper armatures. They are connected with two small brushes, which touch the studs just before their passing under the combs; these brushes play the same part as the two springs a and b (Figs. 82, 83). Two other brushes placed in the middle of the comb of the diametral conductor act like the two springs, c and d.

94. Wimshurst's Machine.-This consists of two identical plates of glass which are mounted on a horizontal spindle so as to rotate in contrary directions, and are provided on the outside with narrow strips of tinfoil arranged radially at equal distances apart (Fig. 89). Two horseshoe-shaped conductors provided with points enclose the two plates at opposite ends of the horizontal diameter, and are in connection with the two poles, and also with two Leyden jars. Diametral conductors, making an angle of from 60° to 90° with each other, are placed, one opposite each plate; they have small brushes at their ends which graze against the bands of tinfoil.

The direction of rotation of the plates is such that a radius, which at any instant is horizontal, becomes parallel to the corresponding diametral conductor by turning through an acute angle. To explain the action, suppose that, by the approach of an electrified body or otherwise, the potential near one end of one of the diametral conductors,-say at the left hand upper part of the figure,—is made higher than elsewhere. A tinfoil sector passing through this region is connected by the conductor with the sector at the opposite end of the same diameter, and the two are, there

fore, brought for the moment to the same potential, by the passage of positive electricity through the conductor from the first to the second. The motion of the plates brings these two sectors simultaneously opposite two sectors of the other plate, which are for the moment connected by the second diametral conductor. This second pair of sectors thus assume the same potential, although one is near a negatively charged sector of the first plate, and the other near a positively charged sector. This equalisation of potentials involves a passage of positive electricity through the

second conductor from the lower, left hand, end to the upper, right hand, end. These actions taking place with all the sectors in turn as they pass the diametral conductors, the result is a continuous succession of positively and negatively charged sectors on one plate, arriving simultaneously opposite the two ends of the diametral conductor applied to the other plate. As we have described the action, the sectors arriving opposite the left hand ends of both conductors would be positively electrified, and those to the right negatively. There would thus be a continuous flow of positive electricity through both conductors from left to right, charging the sectors positively which pass simultaneously through the left hand collecting comb. Similarly, those which pass simultaneously through the right hand comb would be negatively charged. The combs take up the electrification of the sectors that pass them, and there is, consequently, a flow of positive electricity from the left hand comb and discharging knob to the right hand comb, which compensates the transfer from right to left due to the motion of the plates and the action already described. The action of the machine is such as to increase the initial inequality of potential, and it, therefore, acts more powerfully as the motion goes on.

As a general rule, a machine like that of Holtz, where the electricity has to strike across an interval of air, can only begin to work with an appreciable charge. On the other hand, a machine with direct metallic contact or with a brush may start itself, however small the initial charge. It is very seldom that there is not sufficient difference of potentials between the different parts of a Wimshurst's machine to make it begin work.

95. Yield and Energy of a Machine.-By means of Lane's unitjar (§§ 84, 89), it may easily be shown that the yield of inductive machines is proportional to the velocity of rotation, independent of the capacity of the conductor, and of the absolute value of the potential of the poles; it does not alter when one of these is connected with the ground; it diminishes as the difference of potentials between the poles increases. The yield of inductive machines is ordinarily much greater than that of frictional machines. With a Lane's jar with a striking distance of 1 mm., the yield for one turn of a plate machine, 98 cm. in diameter, being taken at 1, that of a Holtz machine of 55 cm. was 0.86. But the former made one turn in a second, and the latter ten turns, so that the yields per second were as 1 : 8.6.

The following numbers will give an idea of the absolute value of

the yield seven turns of a double plate Holtz machine charged a battery, the electrostatic capacity of which was 22,500 cm., so as to give a spark 0.1 cm. in length.

As the capacity of the battery was

22,500 32 × 105

=0.025 microfarads

and since a striking distance of 0.1 cm. corresponds to a potential difference of 4830 volts (§ 73), we get for the charge of the battery

Q=VC=4830 × 0.025 × 10°

-6

=

= 120.75. × 10 6 coulombs,

and for the corresponding work (§ 39)

T=} MV=1× 4830 × 120.75 × 10-6=0.742 joules.

These numbers correspond to seven turns of a machine, which makes ten a second; so that to get the numbers for one second we must multiply by 10 and divide by 7. We thus find that the machine would give per second 0.00017 coulombs of electricity, and 1.06 joules of work.

The power of a machine is defined by the work which it does in unit time. The watt, which corresponds to one joule per second, has been taken as unit of power. We should accordingly say that the power of the machine was 1.06 watts.