PRESENT PROBLEMS 95 



or to travel against the negative current. The anode terminal has a 

 visible discharge which appears to pass from it, and the photographic 

 plate at the anode looks somewhat like a picture of a relief map of 

 the delta formation at the mouth of a river. 



If a conductor be laid upon the plate between the two pin-points, 

 there are then two gaps in the circuit. Each has an anode and 

 a cathode. This conductor may be a metal disk armed with pins 180 

 degrees apart, which face the discharge points. It may be a pencil- 

 mark upon the film or even a spot of reduced silver on the film. The 

 same discharge will start from the cathode terminal of this inter- 

 mediate conductor and will travel slowly in the negative direction. 



With an induction coil giving an eight-inch spark, these ball dis- 

 charges can be formed on the surface of wood. In all cases it is evi- 

 dent that chemical work is being done by the slowly advancing ball 

 or point of light, and it is interesting to observe that it is the cathode 

 discharge only which seems to be active. The reason for this may be 

 partly electrical and partly chemical. The anode terminal of the 

 machine may be grounded on a gas-pipe, and the cathode terminal 

 only armed with a point, and the plate may be placed far away from 

 the machine, connection being made between its cathode terminal 

 and the pin-point on the film, with the same results. It may be added 

 that these plates may be of the most sensitive character, and may 

 be freely exposed to daylight for days before they are used. They may 

 also be developed in the light in a bath not very strongly alkaline. 

 The plate will develop clear, with the discharge tracks dark. The 

 picture will not reverse photographically. It probably would do so 

 if the plate were exposed to direct sunlight w r hile the electrical ex- 

 posure is made. 



With an induction coil having an alternating potential on its 

 terminals, these ball discharges may be obtained from both terminals. 

 They will travel towards each other if on the same plate, but they 

 will not unite. 



In a closed circuit, one part of which is moved across the lines of 

 a magnetic field, as in the case of a dynamo, we must suppose that 

 the positive and negative currents, if both exist, are superposed in 

 that part of the wire in which the electromotive force originates. 

 The currents are superposed at their origin. The same ether machin- 

 ery which urges the positive current in one direction urges the nega- 

 tive current in the opposite direction. With the Holtz machine, we 

 have one half of the machine positively and the other half negatively 

 charged. If the knobs are widely separated, and conductors each 

 armed with the pin-point be led off in opposite directions, each ter- 

 minating on the film of a photographic plate, the cathode will 

 deliver a ball discharge upon its film, while the anode will not. The 

 machine terminal which is not being used may, if desired, be grounded 



