542

SCIENTIFIC RESEARCHES,

When the machine is at work, sparks will pass from the prime conductor to theball, w; and again from the inner side of the ring to one or both of the spotted tubes,which will be brilliantly illuminated, especially if the spindle be touched with thehand, or connected with the cushion by a copper wire. Let now the wheel, w w, begently turned: the spotted tubes will still be illuminated, but instead of showingstationary spiral lines of fire, they will now exhibit the most pleasing spectacle ever be-held in the whole range of electrical illuminations, whose fantastic forms will undergo avariety of changes with the speed of the wheel; and, when the velocity is considerable,the optical illusion creates ideas of a complete disc of electrical light.

This splendid experiment is susceptible of much pleasing variation. If, instead ofhaving the tubes cemented into the central revolving ball, they be fastened to it withscrews in the usual way of screwing balls on the extremities of wires, they may easilybe removed and replaced by other devices, such as tubes of coloured glass spotted inthe same manner, or by slabs of plate glass, spangled in the usual way and varnishedwith different colours. In this way a disc of any coloured light may be composed ofconcentric annular portions, each of a different colour. If, for instance, the faces oftwo revolving slabs of glass were each divided into three equal portions of lines per-pendicular to their edges, and that the inner portion be varnished yellow, the middlered, and the outer portion blue, each portion would form an annulus of its own colour,and the whole would fill up the whole disc. In all experiments with this apparatus,however, the disc, whatever colour or colours it may exhibit, will necessarily appearannular, because of the central brass ball, which, in a darkened room, is a completeblack speck.

By this apparatus differently coloured pieces of glass may be made to combine thelight they transmit, and the composition of colours displayed in the most splendidmanner.

The second experiment is performed by the aid of a magnetic electrical machineand the apparatus represented by Fig. 17, Plate XI. which is an end view.

In all magnetic-electrical machines made in London, the coil wires terminate incylinders concentric with the axis of motion, or spindle, which carries the revolvingcross-piece with its coils; which cylinders are imperfectly insulated from each otherby an intervening cylinder either of box-wood, ebony, or some material of an imperfectconducting character.

In Fig. 17 the central white space represents the end of the inner cylinder, usuallya piece of stout copper or brass wire, to which the inner ends of the coil wires aresoldered. The shaded annulus next this cylinder is the hollow insulating cylinder ofwood, and around it is seen the end of another unshaded annulus representing theouter brass cylinder, to which the outer end of the coil wires are soldered. Thesecylinders fit one another pretty tight, to prevent their moving out of their places.