Page:Popular Science Monthly Volume 63.djvu/452

448 silver plugs. The sensitive material consists of a mixture of metallic filings, five per cent, silver and ninety-five per cent, nickel, being carefully mixed and sifted to a certain standard fineness. In the manufacture of these tubes, great care is taken to make them as far as possible absolutely identical. Each tube when finished is exhausted, but not to a very high vacuum. The tube so finished is attached to a bone holder, by which it can be held in a horizontal position. The object of beveling off the plugs in the Marconi tube is to enable sensitiveness of the tube to be varied by turning it round, so that the small quantity of filings lie in between a wider or narrower part of the gap.

Other ways of adjusting the quantity of the filings to the width of the gap have been devised. Sometimes one of the plugs is made movable. In other cases, such as the tubes devised by M. Blondel and Sir Oliver Lodge, there is a pocket in the glass receptacle to hold square filings, from which more or less can be shaken into the gap.

An interesting question, which we have not time to discuss in full, is the cause of the initial coherence of the metallic filings in a Branly tube. It does not seem to be a simple welding action due to heat, and it certainly takes place with a difference of potential, which is very far indeed below that which we know is required to produce a spark. On the other hand, it seems to be proved that in a Branly tube, when acted upon by electric waves, chains of metallic particles are produced. The effect is not peculiar to electric waves. It can be accomplished by the application of any high electromotive force. Thus, Branly found that coherence may be produced by the application of an electromotive force of twenty or thirty volts, operating through a very high water resistance and thus precluding the passage of any but an excessively small current. Again, the coherence seems to take place in some cases when metallic particles are immersed in a liquid, or even in a solid, insulator. Professor Branly has therefore preferred to speak of masses of metallic granules as radio-conductors, and Professor Bose has divided substances into positive and negative, according as the operation of electromotive force is to increase the coherence of the particles or to decrease it.

It has been asserted that for every particular Branly tube, there is a critical electromotive force, in the neighborhood of two or three volts, which causes the tube to break down and pass instantly from a