HERTZIAN ^YAVE WIRELESS TELEGRAPHY. 449 



conductive to a conductive condition, and that this critical electro- 

 motive force may become a measure of the utility of the tube for tele- 

 graphic 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 instru- 

 ments, 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 increas- 

 ing 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 ascer- 

 tained that there was a complete correspondence between the sensitive- 

 ness of the tubes used as telegraphic instruments and the form of the 

 characteristic curve of current and voltage drawn by the above de- 

 scribed 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 



v=V (l — e"') 



where V and Jc are constants. 



The current through this ball coherer is therefore a logarithmic 

 function of the potential difference between its ends, of the form 



i^ « log (v — V) 



and exhibits no discontinuity. 



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

 VOL. Lxm. — 29. 



