Fig. 16.—Marconi Sensitive Tube or Metallic Filings Kumascope. PP, silver plugs; TT, platinum wires; F, nickel and silver filings.
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 Banly 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. Processor 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 neighbourhood of two or three volts which causes the tube to break down and pass instantly from a non-conductive to a conductive condition, and that this critical electromotive force may become a measure of the utility of the tube for telegraphic purposes. Thus, C. Kinsley (Physical Review, Vol. XII., p. 177, 1901) has made measurements of this supposed critical potential for different "coherers," and subsequently tested the same as receivers at a wireless telegraph station of the U.S.A. Signal Corps. The average of twenty-four experiments gave in one case 2·2 volts as the breaking down potential of one of these coherers or Branly tubes, 3·8 volts for a second and 5·5 volts for the third. These same instruments, tested as telegraphic kumascopes, showed that the first of the three was most sensitive.
On the other hand, W. H. Eccles (Electrician, Vol. XLVII., pp. 682 and 715, 1901) has made experiments with Marconi nickel-silver sensitive tubes, using a liquid potentiometer made with copper sulphate, to apply the potential so that infinitesimal spark contacts might be avoided and the changes in potential made without any abruptness. He states that if the coherer tube is continuously tapped, say at the rate of fifty vibrations per second, whilst at the same time an increasing potential is applied to its terminals and the current passing through it measured on a galvanometer, there is no abrupt change in current at any point. He found that when the current and voltage were plotted against each other, a regular curve was obtained, which after a time becomes linear. A decided change occurs in the conductivity of the mass of metallic filings when treated in this manner at voltages lower than the critical voltage obtained by previous methods. He ascertained that there was a complete correspondence between the sensitiveness of the tubes used as telegraphic instruments and the form of the characteristic curve of current and voltage drawn by the above-described method.
In the same manner, K. E. Guthe and A. Trowbridge (Physical Review, Vol. II., p. 22, 1900) investigated the action of a simple ball coherer formed of half a dozen steel, lead or phosphor-bronze balls in slight contact. They measured the current i passing through the series under the action of a difference of potential v between the ends, and found a relation which could be expressed in the form
where V and k are constants.
The current through this ball coherer is, therefore, a logarithmic function of the potential difference between its ends, of the form