Proceedings, &c, for 1885. 175 



100 amperes. Our lamp resistance will thus be -j 5 ^ = f ohm ; and 

 assuming again a 90 per cent, efficiency, we shall have to make 

 M . + r — 2V ohm, which means a very expensive machine of very 

 heavy wire, and if the lamps are as before — one mile away — our 

 conducting wire must be of 16-in. diameter, and pure copper. 



The E.M.F. required will be a trifle over 50 volts — a very low 

 tension, the shock of which can hardly be felt, and the spark at 

 breaking of contact will be very slight. The points against the 

 parallel system are either the expense of the machine and wire on 

 the one hand, or the inefficiency if the cost of the machine, &c, is 

 kept down. 



In its favour it may be said that it is free from danger, and 

 that one lamp going out hardly affects the rest. This includes the 

 important consideration that any lamp can be turned on and off at 

 pleasure. This is the mode usually adopted, and is the only one 

 that has been practised in Melbourne, a compromise being effected 

 between the cost of the machine and its efficiency. As we have 

 seen above, all the other systems in use must depend on application 

 of these two. A brief reference to two or three of the more 

 important of them must now be made. 



There are two methods which are so closely allied to the funda- 

 mental systems that they must be taken next in order. These are 

 the series-multiple and the multiple-series systems. 



In the one the main current traverses two large main- wires, and 

 these are tapped at intervals by secondary wires, each of which 

 carries a series of lamps. Of course the failure of a single lamp in 

 the series puts that group out, but does not affect any of the other 

 groups appreciably. In the second we have one main wire carrying 

 groups of lamps at various points (the current, of course, being 

 divided at each point, passing through the lamps, and then con- 

 verging once more into the main wire, and so on for each group). 



The advantages are lighter mains, and consequently a larger 

 range, while the disadvantages are the danger, and that a whole 

 group must go out at one time. Thus we shall need group cut-outs, 

 lamp cut-outs, and a current regulator. 



The groups need not be identical, but must each take the same 

 amount of current. In using the series-multiple system we can 

 provide against the failure of any one lamp by means of an idle 

 wire passing from group to group — of course so long as the 

 potential at each end of this wire is the same, no current passes 

 along it, but as soon as it is altered at one end — as by a lamp going 

 out — the whole current for that group passes along it. Of course 

 it is possible to extend the idle wire to the machines, bringing it 

 in between the two. "We are not aware that much has been done 

 in this way yet, but it seems to be a promising field. By using it 

 any part of the whole column may be extinguished. Of course 

 the extra copper required for the idle wire is above that in the 



