C. GENERAL PHYSICS. 197 



sparks, not only by their form, their light, and their noise, 

 but also by other peculiarities, such that a further investiga- 

 tion into their nature has been made by him. Among the 

 isolated points considered in connection with these feeble 

 sparks, Riess states that Wiedemann and Ruhlmann have 

 from special investigations concluded that the quantity of 

 electricity necessary for a discharge is much greater when 

 the discharge takes place from the negative than from the 

 positive electrode. But his own experiments seem to him 

 not to justify so positive an expression, as they can be ex- 

 plained by means of the feeble glimmer that precedes an 

 eruptive discharge. We have, moreover, no experiments 

 which would lead us to think that there is a greater quan- 

 tity of electricity present in a positive than in a negative 

 spark. From experiments made with the Holtz machine, 

 he concludes that the Greater length of the negative elec- 

 trode is not an important condition in producing feeble 

 sparks, but that in these experiments electrodes may be em- 

 ployed of any length whatever. The feeble sparks are, not 

 only in reference to their length, but also to their light and 

 brightness, independent of the composition of the arms of 

 the discharger by means of which they take place. JSIo- 

 natsbericht der Berlin Akad., 1875, 152. 



TELEGRAPHIC GROUND CURRENTS. 



Speaking of the importance of observing the underground 

 or so-called earth currents on electric telegraph wires, Sir 

 William Thomson states that an observation which would 

 be of value for scientific study is to observe the indication 

 of the electrometer at each end of the telegraph line at any 

 time whether during a magnetic storm or not during the 

 day or night. If the line be worked with a condenser at 

 each end, this observation can be made without in the 

 slightest degree disturbing the practical work through the 

 line by simply putting on an electrometer in direct connec- 

 tion with the line, and connecting the outside of the electrom- 

 eter with a proper earth connection, when it may be ob- 

 served, quite irrespectively of the signaling, when signaling 

 is done, as it very frequently is, on submarine lines with a 

 condenser at each end. The scientific observation will bo 

 disturbed undoubtedly by the sending of messages; but the 



