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ELECTRIC LIGHTING.

somewhat complicated, and the current employed difficult tỏ insulate. The apparatus has now been greatly simplified and an unusual volume of light obtained from the tubes, while the current employed is under comparatively low potential.

A Geisler Tube is a glass tube containing two platinum wire terminals passing through the glass and having the air exhausted so as to leave a partial vacuum. Electric discharges or sparks pass between the two wires much more easily in a vacuum than in ordinary air. The passage of the

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discharge causes the air or other gas to become dimly luminous. The color of the light depends upon the kind of glass and upon the gas enclosed. Geisler tubes are sometimes made in beautiful designs. The incandescent lamp is the most common form of the Geisler tube.

ELECTRIC LIGHTING.

Crooke's Tube, or Crooke's Radiometer.-This is a device similar to the "Geisler" tube and produces the most powerful rays known to physicists

The rays, however, do not come from the visible illumination. Prof. Roentgen calls them X rays, for want of any other name. They cannot be seen. They are mysterious and unknown, except that they are proven to exist.

The remarkable property of the X rays is the power to penetrate objects opaque to the human eye, as a block of wood, the human body and thousands of other things. The rays falling upon an opaque object do not render it translucent in the sense that the eye can then see within or beyond it. The manner in which it is proved that the rays penetrate the opaque object is to make a photograph of the interior of the object.

It is quite a different process from ordinary photography. The object to be photographed, a lead pencil for instance, is placed between the Roentgen light and a sensitized plate. The X rays penetrate the wood and whiten the plate. They fail to penetrate the lead within the wood, and that is shown in black relief upon the plate. So with the hand. The hand is put between the light and the plate. The X rays pass through the flesh and whiten the plate, but the rays cannot penetrate the bones, so the bones are shown in black shadow on the plate.

Standard Incandescent Lamp Sockets.-This is a question for immediate agreement among makers. It is thought the

ELECTRIC LIGHTING.

most practical and satisfactory solution is a form of base or mount so inexpensive that when the lamp is destroyed the base may be allowed to go with it as in itself a thing of no value. In view of the fact that high tension lamps are in all probability to be the lamp of the future, the question of a safe socket acquires vast importance. Porcelain will no doubt come largely into use for this purpose, which will greatly modify the conditions governing the selection of both a base and socket. See Figs. 157, 158, 160.

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Whatever the source of electrical energy and whatever devices may be used to convert it into its destined purpose the transference is made by the means of wire.

Gas, water, etc., are distributed for all sorts of purposes and the laws governing them are similar to those governing electricity. Each trade has, of course, some elements which have to be considered that are neglected in others, but the main laws are common to all and do not require different interpretations except possibly in degree.

NOTE. Size for size, a thread of spider silk, it is said, is decidedly tougher than a bar of steel. An ordinary thread will bear a weight of three grains. This is just about fifty per cent. stronger than a steel thread of the same thickness.

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