Page:Popular Science Monthly Volume 63.djvu/453

Rh 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 thfe 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

and exhibits no discontinuity.

The inference was drawn that the 'resistance' is due to films of