6i6 



NA TURE 



[October 19, 1905 



tribution of power ever long distances? Practically, every 

 electrical engineer will at once reply, alternating, of course. 

 Well, I am going to preach heresy! I say direct current ! 



The alternating current has' undoubtedly the great 

 advantage that a motor can be constructed with no rubbing 

 electric contacts, every wire may be permanently soldered 

 in position, a condition of considerable importance in dusty 

 places like mines. Here is such a motor — the first poly- 

 phase motor ever sent from America to Europe, the first 

 ever seen in lireat Uiitain, constructed seventeen years 

 ago by Tesla with his own hands, when he was too poor 

 to employ a workman. 



-Another advantage possessed by an alternating current 

 is that an alternating current dynamo can be constructed 

 to produce a large horse-power at a high voltage, and 

 further, as we have already seen, this alternating voltage 

 can be transformed into a still higher one without the use 

 of moving machinery. 



This is one of the five largest dynamos in the world. 

 Its size you can better estimate by looking at the ring stand- 

 ing on end, now projected to the left. The latter is the 

 stationary portion of a 5000 horse-power horizontal shaft 

 dynamo, while the photograph to the right is that of a 

 vertical shaft machine of double that power, viz. a dynamo 

 that can develop 10,000 horse-power at a pressure of 

 11,000 volts. Fifteen years ago, Ferranti — the Brunei of 

 electricity — spent a mint of money constructing some of 

 the parts of a 10,000 horse-power, 10,000 volt alternator, 

 which were, however, never put together. This dynamo 

 projected on the screen stands complete, with its four 

 sisters, in the Canadian Niagara Power House, and the 

 tests already made show that its efficiency reaches the 

 extremely high value of 98-2 per cent., that is, 1.8 per 

 cent, of the power developed is suflicient to cover all 

 losses. Ferranti 's dream is more than realised, and the 

 old story is repeated. We break up the pioneer leviathan, 

 the Great Eastern steamship, as a great unwieldy giant 

 very weak in its knees, a little later we build the Baltic, a 

 third as large again, and with twice the engine power. 



Without any transformation at all, these dynamos will 

 economically drive machines some miles away, and, with 

 the pressure transformed up from 11,000 to 60,000 volts, 

 power will be distributed in Toronto, 85 miles awav from 

 the falls. 



Contrasted with this, no single large direct current 

 machine has ever been constructed to generate more than 

 about 3500 volts, and no means is known for efficientlv 

 converting a direct current voltage into a higher, or a 

 lower one, without the use .of moving machinery. 



So far, then, my case seems weak ! The advantages of 

 using great electric pressures we have seen. Are there 

 any disadvantages? This is a disadvantage, the risk of 

 piercing the insulation ! See how thick the insulating 

 material has to be on cables, how far apart the conductors 

 have to be placed, even when the cable is intended for 

 only 10,000 volts. But does this consideration supplv any 

 argument for or against the use of one kind of current 

 rather than the other? Small current and high pressure 

 must be used for the economical transmission of power 

 over long distances, whether the current be alternating or 

 direct, I agree; but, ladies and gentlemen of the jury, I 

 submit that, while from the point of view of economic 

 transmission, 60,000 volts alternating means e.\actly the 

 same as 60,000 volts direct, from the point of breakdown 

 of the insulation, 60,000 volts alternating is as bad as 

 85,000 volts direct, indeed may be worse than 100,000 

 volts direct. For an alternating current consists of waves 

 like the waves of the sea. In a storm, the waves may 

 be running mountains high, and yet the average depth 

 of the sea remains the same as in a calm. But what does 

 it benefit the poor passengers, when tossed helplesslv back- 

 wards and forwards in their berths, and feeblv calling 

 " steward," to be assured that, although the waves be 

 peaked, and the maximum elevation large, the square root 

 of the mean square of the amplitude of oscillation is quite 

 consistent w-ith perfect internal tranquility? And so feels 

 the poor insulating material — the mean electric pressure 

 may not be very large, and yet the crests of the waves 

 may be so high, and the troughs so low, that its strength 

 cannot stand the electric tossing. 



Each of those waves of electric pressure on the diagram 

 NO. 1877, VOL. 72] 



gives the same reading on a voltmeter, but the peaked on,e 

 has far more destructive action than the flat topped one. 



But there are other disadvantages in the use of 

 alternating current. This coil of wire represents one of 

 the conductors which, when unwound, might join two 

 places, the one where incandescent lamps (for example) 

 have to bo made to glow, and the other where is the water- 

 power which drives the dynamo that generates the current. 

 If a direct pressure of too volts be applied at one end of 

 the system, the lamps at the other end glow brightly, as 

 you see, whereas if now I apply an alternating pressure, 

 although of exactlv the same value, the lamps are quite 

 dull. 



The explanation of this striking difference is that in 

 such a case only a fraction of the alternating pressure is 

 used in making the lamps glow, the remainder being 

 employed in maintaining a rapidly reversing magnetic 

 field. " 



This magnetic effect — this self-inductive effect as i,t is 

 called — is .small if the going an^ return condqctors be 

 straight, short, and near together. But if the distance 

 over which the power is to be transmitted be long, the 

 wires obviously cannot be short, and if to obtain economy 

 high electric pressure be used, the wires cannot be put 

 very near together, since that would lead to a brush dis- 

 charge through the air from one conductor to the oth^r. 

 producing leakage. 



Indeed, the minimum distance that must separate the con- 

 ductors has to be increased very rapidly with the pressure 

 unless their diameter is greatly increased at the same time. 

 The table gives this minimum distance for conductors 

 i/ioth, 2/ioths, and 4/ioths of an inch diameter re- 

 spectively, and it will be seen that increasing the thick- 

 ness of the wire greatly diminishes this minimum. For 

 instance, at 80,000 volts, doubling the thickness of the 

 wire from i/5th to 2/sths of an inch diminishes the mini- 

 mum distance from 62 feet to 13^ inches. 



Johannesburg. 



Elevation, 5689 feet, January, 1905. Barometer, 



24.3 inches. Temperature, 9t°-S F. 



Minimum distance that must separate two parallel ivires 



to prevent the starting of a Brush Discharge. 



It must, of course, be remembered that these are mini- 

 mum distances, and that the distances apart at which- the 

 wires have actually to be fi.xcd in practice are much 

 greater. 



But that is not the whole indictment against the use 

 of alternating current for long distance transmission. 

 Leakage from wire to wire can be rendered small, but 

 still, if the current be alternating, it always flows along 

 the wires, even if all the apparatus at the distant end 

 be entirelv disconnected from them. Let me show vou 

 this. 



I apply a direct pressure of 100 volts, and no current 

 enters the transmission line, for it is well insulated along 

 its length and at its ends. I apply instead an alternating 

 pressure of the saine value, w-ithout making any other 

 change, and you observe a verv perceptible current. The 

 very first thing that struck Ferranti when he commenced 

 transmitting power with alternating current at 10,000 volts 

 pressure, from Deptford to London, was that the current 

 flowing into the system at Deptford was as large during 

 the daytime, when practically no lamps were turned on 

 in London, as during the evening, when manv were glow- 



