94 PHYSICS OF MATTER 



opposite directions within the conductor which joins its coatings. 

 The middle point of the conductor is last reached by the discharge. 

 If the discharge is maintained and a steady current is finally pro- 

 duced, this current must apparently consist of positive and negative 

 electricity flowing in opposite directions. 



If air be pumped out of one boiler and into another, two kinds of 

 pressure are thus generated. If these pressures are added together, 

 by connecting the boilers by means of a conductor, these pressures 

 are added together, and both disappear. If we tap these charged 

 boilers, the discharge from one will attract, and from the other will 

 repel, an uncharged testing sphere. If the testing sphere be itself 

 charged, we shall find that like charges repel, if both are positive, and 

 attract, if both are negative. 



It is unnecessary here to enlarge upon the well-known differences 

 between the positive and negative terminals of an exhausted tube. 

 All of these phenomena will finally be helpful in arriving at the nature 

 of the difference between positive and negative electricity. But I will 

 refer to certain phenomena which do not seem to be so well known. 

 Every one is familiar with the small points of light which may often 

 be seen dancing in a crazy fashion over the cathode knob of the Holtz 

 machine. A similar appearance can be seen on the negative carbon 

 of a direct current arc, and in the negative bulb of the mercury vapor- 

 lamp. These points of light may be made to pass from the cathode 

 knob of the Holtz machine to the surface of a photographic dry- 

 plate, exposed in open daylight. 1 Separate the knobs so that no 

 spark will pass. Place the plate near or between them. Connect the 

 knobs with two small metal disks, each armed with a pin-point, so 

 bent that it makes contact with the film. The point of the pin may 

 rest upon the short mark of a lead pencil, drawn upon the film, the 

 pins pointing towards each other on the plate. Points of light, like 

 the so-called ball-lightning discharges, will come from the cathode 

 terminal and successively travel slowly over the plate, leaving a black- 

 ened trail of reduced silver behind. By means of a lead pencil held 

 in the hand with the point near the cathode pin-point, these dis- 

 charges may be induced to make their appearance on the film, and 

 may be deflected into various directions after they have appeared. 

 When left to themselves these minute specimens, of what may per- 

 haps be called ball-lightning, tend to follow the lines of the field, but 

 their paths are somewhat affected by the paths of prior discharges. If 

 one of these points of light is seen on the pin which arms the cathode 

 terminal, there will usually be none upon the film of the dry-plate. It 

 may be brought upon the plate by holding a pencil-point near it. 



These ball discharges come from the cathode and travel to or 

 towards the anode. They cannot be induced to come from the anode, 

 1 Transactions of the Academy of Science of St. Louis, x, no. 6. 



