448 POPULAR SCIENCE MONTHLY. 



silver plugs. The sensitive material consists of a mixture of metallic 

 filings, five per cent, silver and ninety-five per cent, nickel, being care- 

 fully mixed and sifted to a certain standard fineness. In the manu- 

 facture of these tubes, great care is taken 

 to make them as far as possible absolutely 

 identical. Each tube when finished is ex- 

 hausted, 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 hori- 

 FiG. 1. MARCONI SENSITIVE ^^^^.^j positiou. The obJect of beveling off 



Tube ok Metallic Filings Kum- ^ _ '' = 



AscopE. pp, silver plugs; TT, the j^lugs in the Marconi tube is to enable 

 platinum wires; F, nickel and ^j^^ gensitiveness of the tube to be varied by 



Sliver filings. •• 



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, ojDerating through a very high water re- 

 sistance 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 tul)e, 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 non- 



* This device of making the inter-electrode gap in a tubular filings coherer 

 wedge-shaped has been patented again and again by various inventors. See 

 German patent No. 116,113, Class 21a, 1900. It has also been claimed by M. 

 Tissot. 



