6i4 



NA TURE 



[October 19, 1905 



low pressure wheel along an open stream, and flows away 

 again also along an open stream, no expense has to be 

 incurred in laying down large pipes. If, however, it were 

 necessary to distribute much power over considerable 

 distances through a pipe conveying such low-pressure 

 water, the pipe would not only have to be long, but of 

 large cross-section, and, therefore, very bulky and costly. 

 For example, this model is a full-size representation of the 

 transmission of only one horse-power with low pressure. 



On the other hand, if the water possesses considerable 

 head, the transmission pipe may be of small diameter. In 

 this second model the three-cylinder pump produces a 

 pressure of 425 lb. per square inch, exactly the pressure 

 used in the hydraulic transmission of power down the 

 shaft of the Rietfontein Mine, and with that pressure less 

 than four gallons of water flowing per minute through 

 this three-quarter inch pipe gives as much power to this 

 turbine as would be delivered by 825 gallons pouring per 

 minute over this water-wheel four feet in diameter. 



The water pressures in these two illustrations bear about 

 the same proportion to one another as the electric pressure 

 in the Lauffen-to-Franlifort transmission bears to the 

 electric pressure usually maintained between the terminals 

 of a lamp in Johannesburg. 



The value of using pressure water is grasped when you 

 realise that at the Rietfontein Mine, by circulating about 

 85 gallons of water per minute, at 421; lb. pressure per 

 square inch, through a pipe 16 square inches in cross- 

 section, not only is the circulating water all returned to 

 the top of the mine, but in addition 144 gallons are 

 pumped up per minute from a depth of 546 feet through 

 a pipe 38J square inches in cross-section. 



The water supplied by the London Hydraulic Power 

 Company at 1700 feet head, although not filtered, costs 

 nearly four times as much per gallon as the filtered water 

 furnished by the Metropolitan Water Board. In England 

 dirty pressure water is a relatively costly commodity, 

 sparkling drinking water a relatively cheap liquid. In 

 Johannesburg, on the other hand, until quite recently, the 

 charge for drinking water was ten shillings a thousand 

 gallons, plus two-and-six a month for meter rent, or about 

 twenty times the London rate — the temptation to drink 

 other things in Johannesburg must have been very great. 

 Now, since the establishment of the Rand Water Board, 

 it is six shillings a thousand gallons, which, without meter 

 rent, is still ten times the London price, so that liquid 

 with a head in London is still cheaper than plain drinking 

 water here. 



In the distribution of power, current and pressure are 

 equally important. It is not merely because, even this 

 month, August, after a phenomenally dry season, about 

 5.000,000 gallons of water are rushing per minute over the 

 Victoria Falls, but it is because this water also thunders 

 down about 380 feet that these falls are a potential source 

 of power. 



The Howick Falls, near Pietermaritzburg, have nearly 

 as much head as the Victoria Falls, and twice as much as 

 Niagara, while a syphon of soda water, when the gas is 

 first pumped in, holds its head higher than any of the 

 three. But, although in Johannesburg you probably pay 

 a shilling for a syphon of soda water as an energy-pro- 

 ducer in man, it is not worth 1/ 10, 000th part of a penny 

 as an energy-producer in a turbine, there is so little of it 

 — only a pint and a half. 



Probably, like myself, you have heard vague comparisons 

 made between the power of the Victoria and the Niagara 

 Falls. Now, what is the true comparison ? 



The flow at Niagara varies at different times of the year 

 from about 62 to 104 million gallons per minute. At the 

 Victoria Falls the flow can be as little as one-twelfth of the 

 smaller number — for it is so now ; and some authorities, 

 well acquainted with the spot, sav that at the end of another 

 three months the flow will only be half of even that. The 

 mean available drop at Niagara is about 160 feet ; at the 

 Victoria Falls about 380 feet. Hence, while the minimum 

 Niagara flow represents about 3 million horse-power, the 

 present Victoria flow represents about 580,000 horse-power, 

 or only about one-fifth of the Niagara flow. Further, if 

 those who predict the flow of the Zambesi sinking to some- 

 thmg like 2^ million gallons per minute in November are 

 true prophets, the Mctoria Falls will then only give out 

 NO. 1877, VOL. 72] 



about 300,000 horse-power, or, say, one-tenth of the 

 minimum that Niagara produces. 



In all that precedes, I have taken the full power of the 

 direct drop in each case; that is, I have assumed in each 

 case the intake to be close to the main drop, and I have 

 deducted nothing for inefilciency of machinery. 



Now, how exactly does the efticiency in the electric 

 transmission of power depend on (1) the pressure, (2) the 

 power transmitted, (3) the length of the transmission line, 

 and (4) the resistance of the conductors composing it? 



The very simple approximate formula connects these 

 quantities : — 



Percentage! 



loss of I ^ Horse-po wer tra nsmitted ^ ^^^^^ ^ 

 power on 3 (thousands of volts)- 



the road. 



/ Resistance 

 I per mile of all 

 ithe conductors 

 ! in parallel. 



This formula tells us that as long as the electric pressuie 

 is limited to some 10,000 or 11,000 volts — a pressure boldly 

 used as early as 1897 by the Rand Central Electric Works, 

 and at the Moodie Mines, near Barberton, but the onr 

 that is still the maximum sanctioned in Great Britain — it 

 will not be possible, even with a pair of conductors ol 

 good copper, each as thick as the one I hold in my 

 hand, viz. three-quarters of an inch in diameter, to 

 transmit more than about booo horse-power, or to transmit 

 that power more than about 10 miles, without the loss on 

 the road exceeding 10 per cent. 



The actual efficiency will, of course, be less than 90 per 

 cent., since there will be losses also in the machinery at 

 each end of the transmission system. 



If, however, the electric pressure be doubled, that is, 

 raised to 20,000 volts, then through this pair of con- 

 ductors (kindly put up by the Transvaal Technical 

 Institute, to bring power from their dynamo room to this 

 hall), which are not much more than one-fifth of the 

 cross-section of the former, and therefore not much more 

 than one-fifth of the cost, as regards copper, we can 

 transmit 2700 horse-power 23 miles, and still only lose 

 10 per cent, on the road. 



Now Brakpan, where is the generating station of the 

 Rand Central Electric Works, is almost exactly 23 miles 

 from Johannesburg. Six wires come thence to Johannes- 

 burg, three of which may be likened to the going con- 

 ductor, and three to the return in a two-wire system like 

 this, also any three of those wires have a joint cross- 

 section rather larger than three times the cross-section of 

 this. Hence, with 20,000 volts, about 8000 horse-power 

 could be sent to Johannesburg from Brakpan through the 

 existing wires with only 10 per cent, loss on the road, or 

 about 3400 horse-power (which is rather more than the 

 entire maximum output of that generating station on any 

 occasion last year) could be sent with only 4 per cent. loss. 



I should have liked to show you this e.xperimentally, 

 but Mr. Reunert, Principal Hele .Shaw, and Prof. Dobson, 

 who, since my arrival, have so kindly put themselves to 

 so much trouble to give e.xpression to my wishes, might 

 have thought me a little exacting had I asked for a lecture 

 hall big enough to include a transmission line from 

 Brakpan ; and so, instead of this pair of conductors con- 

 necting two places 23 miles apart, I am going to employ 

 a pair of extremely fine wires, each less than i/iooth of 

 the diameter, that is, less than 1/ 10, 000th of the cross- 

 section — so fine, in fact, that you cannot see them. 



Switch on the current, more than 100 lamps glow. Now 

 think of a wall of lamps ten times as high, then ten times 

 as wide, and then six times as big as all that, and you will 

 have 2700 horse-power ; and that is the power which, put 

 into this pair of wires 23 miles away, say at Brakpan, with 

 this pressure of 20,000 volts, will cause about 2400 horse- 

 power to come out at Johannesburg. 



This experiment of transmitting five horse-power across 

 the hall is the nearest approach to wireless transmission 

 of power that I have ever seen. But there are wires, 

 although invisible, for if I make them touch at one point 

 with this long stick a flash occurs above your heads, and 

 the glow lamps on the platform go out. 



I directed your attention to the fact that in 188S the 

 male population of Johannesburg was 2000. By 189(1, 

 according to the census taken that year, it had grown to 

 32,387. Now, curiously enough, in 1897 two transmissions 



