slight AV they will act inductively upon the carriers that come opposite to them. Now, at one point in their course, as the inner cylinder revolves, each pair of carriers as c, d, are connected with one another through the metallic piece m n, whose extremities are provided with fine wire brushes. In consequence of this, c1

1

will pass on charged —, and c', will pass on charged +.

1

When the two carriers come opposite to the other armatures, A' and A respectively, they make contact with these through the metal pieces B' and B; these pieces reach round the ends of the cylinders, and have wire brushes at their extremities. The question now occurs, will these carriers charge the armatures to greater + and potentials as did the carriers in §§ 3 and 4? The difficulty here arises from the fact that the carriers do not now pass into the interiors of the armatures, and so do not give up all their charge.

We may perhaps reason somewhat as follows. The carrier c1, while still uncharged, is brought very near to the armature A; and we may assume that it is thereby raised to at any rate more than half the potential of A. That is, it is raised to some potential m V, where m is greater than . It is then brought to zero-V by connection with the opposite carrier. It must therefore have received a charge that would, in the absence of A, have lowered it to Next it comes into the same position with respect to A'; and would therefore, if uncharged, have been lowered to a potential - m V. Being charged, however, it will acquire on the

m V.

whole a potential of

m V

-

m V, or 2 m V. But we have supposed m to be greater than . Hence - 2 m V is numerically greater than V; and therefore electricity will flow from electricity in the other direction; and hence, finally, the potential of A' becomes lower than before, or its

armature A' to carrier c1, or

V is numerically increased.

A like action will meanwhile have taken place with respect to c', and A.

So far then we have shown how the armatures are raised to, and maintained at, a great ▲ V.

Now consider the brass pieces M and N, which are represented as furnished with combs and as having an air-break pq between them. (It must here be noted that the 'combs' with which M N and m n are provided should in reality run parallel to the axis of the cylinder. They are drawn as in the diagram so that they may

be seen; whereas strictly they should be drawn 'end-on.') Both the metal carriers, and the varnished surface of the glass which serves as a continuous series of insulated carriers, will act inducelectritively upon the brass combs; + electricity from M, and city from N, will pass convectively from the points of the combs on to the carriers and glass surfaces, tending to reduce the regions that lie opposite to the two combs to a smaller AV; while the 'repelled ' and charges will give a stream of sparks across the air-space pq.

If there be no break at pq, and if the combs have very sharp points, the opposite pairs of carriers and of glass surface will be reduced to almost the same potential; and the piece m n will be idle, M N taking its place. If there be a break at pq it requires a certain ▲ V to give a discharge across this break; and the opposite pairs of carriers cannot certainly be reduced to anything less than this ▲ V; so that mn will now act as above described. The limit to the striking distance p q is given by the A V of the armatures ; the AV between M and N being always less than this. When the distance pq is too great, M and N are idle.

The action, or idleness, of the pieces m n and M N can readily be detected by working the machine in the dark; the presence or absence of glows and brush-discharges giving the required information.

The combs with which the piece m n is provided thus play with respect to the glass surface the same part as the brushes play with respect to the metal carriers.

§ 7. The Holtz Machine.-The Holtz machine was invented before the Voss; but, as its theory is less simple, we have chosen to discuss it later.

The main differences between this machine and those that we have discussed above are that here the armatures are charged inductively, and not directly, by the carriers; and that we have as pairs of carriers' no metal pieces, but simply the diametrically opposed portions of the varnished glass surface.

Fig. i. gives a general view of the simplest form of Holtz; and this figure will serve, mutatis mutandis, to give an idea of the Voss. The larger and hindermost plate A A is fixed. In it are cut two windows F and F'; and along the edges of these windows are pasted the armatures p and p', consisting of two sheets of tin

foil or of varnished paper. From the armatures project two pointed tongues of paper, n and n'; these touch lightly the back surface of the smaller revolving plate B B (which is the nearer to us in the figure), the points of contact being opposite to the windows F F' cut in the fixed plate. The front plate B B revolves as indicated by the arrows.

On the nearer side of the front plate B B. and opposite to the armatures pp', are the combs O O' of the conductors CC'. These

conductors are put into contact by means of the pieces K and K', until the action resulting from the revolution of B B has increased the small initial A V of the armatures to a sufficient magnitude. When this has been effected, an air-space may be introduced at rr', when a discharge will take place across.

As in all induction machines, this discharge will be nearly continuous, and will consist of a luminous brush accompanied by small sparks; unless, by connection with the condensers H H'

or by some other means, the capacity of the prime conductors be greatly increased. When this is done, the character of the discharge alters; it now takes place at intervals in the form of a dense and brilliant spark.

In fig. ii. we give a representative diagram. The lettering of this, similar to that of the figure to § 6, differs from that of fig. i. above; but the reader will find no difficulty in recognising

the various parts. Here we

have the sections of two co-axial cylinders, of long strips of tinfoil which form the armatures, and of long open strips which form the windows cut in the outer fixed cylinder. As in the case of the Voss, the combs should run parallel to the axis of the cylinder, i.e. to the edges of the windows and armatures; they should have been viewed end-on instead of as here drawn.

As in the other machines discussed above, there is given initially to A A' some slight ▲ V. The conductors are then put into contact, no'air-space being left atq; and the inner cylinder is turned as indicated by the arrows. The opposite pairs of what we may style 'glass-surface carriers' are thus charged inductively, as usual.

Let us consider the

-

charged glass surface that comes opposite to B'. In virtue of its charge, and of the presence of the charged armature A', this portion of glass surface will acquire a negative potential greater in magnitude than that of A' and B'. There will therefore be a field of force running from the armature A' towards the 'carrier,' since this is at a negative potential of greater magnitude. Hence we shall have + electricity passing off the tongue B' on to the back surface of the inner cylinder; while the armature A' is consequently left at a negative potential numerically greater than before.

When our 'carrier' glass surface comes opposite to the comb M, it will be charged as usual; and the charge that was

'bound' at the back of the cylinder will become 'free.' When the same portion comes in turn opposite to B, this tongue will be charged to a higher + potential, in part directly by this free + charge on the back of the inner cylinder, and in part inductively by the charge on the inner surface of the cylinder, in the manner described above.

When the AV between A and A' is great enough, an airspace may be introduced at pq; and sparks will now strike

across.

But it is to be noticed that if this air-space be great, requiring a large ▲ V to drive a discharge across it, then the charges on the glass surfaces will fail to be

mn, furnished with combs at the ends, may be introduced, as shown in fig. iii. This form of Holtz is more like the Voss. The piece mn with its combs will now insure the reversal of charge upon the glass surfaces; and, if the gap pq be great, but not too great for the insulation of the machine, the AV of the armatures A A' will increase until a spark can strike across pq.

Notes on the Voss and Holtz.-(i.) It may be remarked that in both the above machines it is only a small portion of the revolving surface that is employed to raise the armatures to, and maintain them at, the high ▲ V required. In the Voss this part is played by the metallic carriers; these in the actual machine are small metallic discs which bulge out at their centres sufficiently to make contact with the brushes at # and n. In the Holtz the same function is performed by that ring of glass surface which lies in the immediate neighbourhood of the circle traced out on the revolving plate by the sharppointed tongues.