ELECTKICITY IN RELATION TO HORTICULTURE. 



17 



used for heating purposes rapidly corrode the metal work of the 

 machine. To obviate this I have now had a chimney inserted to lead 

 off the heated air as it passes along the corrugated iron base. This 

 will probably also increase the efficiency of the machine, in that it will 

 prevent a rise in the general temperature of the shed in which the 

 apparatus is housed. The machine is at its best when the temperature 

 of the plates is somewhat above that of the air around. 



For experiments upon a larger scale, or when expense is not the 

 Hmiting factor, then the newer system of generating electricity which 

 I have elsewhere alluded to as the " Lodge-Newman " system is much 

 to be preferred. Though more expensive in initial outlay, this system 

 would probably be found economical in all cases where continuous 

 running over long periods is required, and where there is available 

 som.e ordinary supply of direct current at any of the usual voltages. 



Where such a current is not available, then the outlay is increased 

 by the necessity of generating current by means of a motor and direct 

 current dynamo. As a set-o££ to this consideration, as the apparatus 

 will only require some five to ten amperes of current at the outside, 

 there would probably be available from the dynamo a considerable 

 amount of current that could be used for other purposes, e.g. to charge 

 batteries, for electric lighting, etc. 



The direct current is taken through a mercury " break " and the 

 primary of a large induction coil. The result is, of course, high 

 voltage electricity generated in the secondary of this coil in alternating 

 surges of positive and negative electricity. But between the secondary 

 and the outside circuit there intervene some of the valves invented by 

 Sir Oliver Lodge. These valves will only permit the electric charge 

 to jump across them in one direction. They can therefore be arranged 

 so as to permit only the positive surges of electricity to get across or 

 only the negative. In this manner the outside network of wire may 

 be charged continuously to a high voltage with either positive or nega- 

 tive electricity. 



The actual voltage on this outside current can be tested at any 

 time by seeing through what distance of air a spark will jump from 

 a wire connected to this network to another wire connected to earth. 

 In actual experiments the distance between the two conductors in this 

 spark gap may vary between half an inch and an inch and a quarter — 

 distances which indicate pressures running at their higher limit to over 

 one hundred thousand volts. 



The great advantage that this method of charging the overhead 

 system of wires possesses, is that the induction coil produces so much 

 greater quantities of electricity than the electrical friction or influence 

 machines. As a consequence, though the leak from the system of 

 wires outside may be considerable, yet the quantity of electricity pass- 

 ing from the coil may be sufficient to keep up the necessary high 

 voltage upon the wires. In my experiments with the other method, it 

 has often been my experience that the addition of a few discharge points 

 to my overhead system has been sufficient to bring down the potential 



VOL. XXXVII. . C 



