APPENDIX 427 



Arc Lamps with Alternating- Current. Where the current is 

 alternating, it is customary to place both carbons vertical, both of 

 equal size and, as a rule, both cored. This is the plan recommended 

 in every text-book I have come across, but, after many years' 

 practical experience with alternating currents, I have come to the 

 conclusion that the arrangement is wrong. The best effect (in my 

 hands at any rate) is to slant the carbons as for continuous current, 

 to have the carbons in line instead of the upper one slightly behind, 

 the upper one still to be cored and the lower one solid, and the 

 upper one slightly the largest, though the difference in size should 

 not be so much as with the continuous current, as in this case the 

 upper one would only burn away faster because of the soft core, no 

 difference in the rate, because one carbon is positive and the other 

 negative, having to be reckoned with when the current is 

 alternating. 



The chief disadvantages of the alternating current are : 



(1) The feebler light obtained (being only above 50 per cent.) as 

 compared with that given by the same amount of continuous 

 current. 



(2) The tendency of the arc to dance round the carbons, instead 

 of keeping on the frontal line of the two, as it should do, for a 

 steady light. 



(3) The humming noise produced by the vibrations of the 

 current in the arc itself. 



For this last there is no real remedy. The objection can be 

 reduced to a minimum by placing the whole lantern on a thick 

 layer of solid felt ; a pad about 1 inch thick, which can be 

 obtained from most saddle makers, has a marvellous effect in 

 reducing the noise. Much can also be done by reducing the 

 voltage to the right amount, and this fortunately also largely tends 

 to prevent the dancing about of the arc spoken of. 



The alternating current, unlike the continuous, can easily and 

 efficiently be " transformed" to any required voltage, which point 

 is of so much value in commercial work that it is the real reason 

 why these currents are so often employed. The current, however, 

 must not be transformed down to the 45 volts or so necessary for 

 lantern work, or it will be found impossible to keep a steady light. 

 It is obvious that when the whole of our energy is being absorbed 

 in the arc itself, any variation in the length of this arc will cause a 



