November 25, 1909] 



NA TURE 



yiolors. 



In three-phase motors for railway work, speed regula- 

 tion has hitherto been obtained either by some kind of 

 cascade arrangement or by changing the number of poles. 

 In either case the rotor' has slip-rings, a complication 

 one would gladly avoid. This is now possible, thanks to 

 an ingenious design worked out by Mr. Aichele, the chief 

 designer of Messrs. Brown-Boveri. The motor has been 

 applied in their latest Simplon locomotives. Its rotor is 

 simply a squirrel-cage, and has no slip-rings and no out- 

 side electrical connections whatever. The stator has two 

 distinct windings, one for i6 and the other for 12 poles, and 

 each winding can by means of a pole-changer be so 

 grouped as to produce half its normal number of poles. 

 There are thus four normal speeds possible, correspond- 

 ing respectively to 16, 12, 8, and 6 poles, or to a train 

 speed of from 26 to 70 km. per hour. 



A remarkable improvement in single-phase motors has 

 been devised by Mr. Deri, and practically developed by 

 Messrs. Brown-Boveri. Mr. Deri's motor is a " repulsion- 

 motor," with movable and fixed brushes. The effect of 

 shifting the former is analogous to changing the im- 

 pressed voltage on an ordinary continuous-current series 

 motor, and thus by adjusting the brushes the torque and 

 speed may be regulated. This property renders the Deri 

 motor valuable in all cases where delicate speed regulation 

 is essential. It is largely used for working passenger 

 lifts and other hoisting machinery, and also for driving 

 ring-spinning frames, the speed regulation in the latter 

 case being automatic. The result of automatic speed re- 

 gulation is an increased output from the ring-spinning 

 frames. Another application is for electric railway work- 

 ing, to which I shall refer later. 



The Electric Transmission of Power. 

 There has been a considerable development in this branch 

 of applied electricity in late years, but the development 

 has been on different lines in different countries correspond- 

 ing to their various topographical, industrial, and com- 

 mercial conditions. With us it is not so much a question 

 of carrying power a long way as of distributing large 

 amounts of power at numerous points within a restricted and 

 densely populated area. In so-called water-power countries 

 the distance between the source of the power and the 

 points of its delivery is very much greater than in England, 

 and hence the necessity of using much higher pressures in 

 the transmission lines. In raising the pressure a limit 

 is eventually reached at which dispersion of power becomes 

 serious. This critical potential difference in virtual kilo- 

 volts is : — 



Kv = 22i5_Y "^ Viogi 



o-s + r\i+o-oiy,/J ^ r 



Here h is the barometric pressure in mm. of mercury, r is 

 the radius of the wire in cm., i the distance between the 

 two wires in cm., and v is Mershon's " vapour product," 

 namely, the pressure of saturated steam in mm. of mer- 

 cury at the given temperature multiplied by the relative 

 , . ,. , . actual moisture 



humidity, or the ratio — ., , -. 



possible moisture 



The protection of power lines against pressure surges 

 due to atmospheric or other causes is a very important 

 matter. 



It is well known that the connection of an underground 

 cable with an overhead line constitutes a special danger 

 to the cable from atmospheric discharges. To protect the 

 cable, Mr. Semenza, of Milan, uses a kind of gigantic 

 Faraday cage surrounding the point where the overhead 

 lines are connected to the cables by transformers. The 

 iron parts of the structure are earthed, the roof and the 

 window-frames are of iron, and under the plastering of 

 the walls there is iron netting. If a capacity and induct- 

 ance tuned to somewhere near the frequency of the surge 

 arc placed in series and connected to line and cage, a cur- 

 rent of that particular frequency will flow to earth as if 

 the connection were direct. Even if the frequency were 

 only approximately that to which the set was tuned, the 

 reactance would not be excessive and the protection would 

 be sufficient. Thus a set tuned to i million frequency 

 would at 10 million frequency have a reactance of 158 

 ohms and at 100,000 frequency a reactance of 165 ohms. 

 NO. 2091, VOL. 82] 



A set tuned to 100,000 frequency would at 20,000 frequency 

 have a reactance of 192 ohms. A set for i million fre- 

 quency may conveniently be formed of two Moscicki con- 

 densers in parallel, having together a capacity of 001 mf. 

 and an inductance of 2-54 microhenry. The latter is 

 obtained by two turns of 2 mm. copper wire 50 cm. in 

 diameter. A set for 100,000 frequency would require eight 

 condensers in parallel and a coil of ten turns. For the 

 ordinary working frequencies up to 50 either set has of 

 course a practically infinite reactance, that is to say, it 

 has no effect on the power current. The Milan trans- 

 lating station has been at work now for about two years 

 with perfect success. It should be noted that the system 

 not only protects against lightning discharges, but against 

 any abnormal rise of pressure, in so far as this is caused 

 by a high-frequency surge. 



Whilst on the subject of safety devices in connection with 

 power transmission, I must refer to another recent in- 

 vention, the object of which is the prevention of the 

 infiltration of high-pressure current into low-pressure lines. 

 That such a device is urgently needed is shown by the 

 lamentable accident which happened last August in Olgiate, 

 where several persons were killed by contact with nominally 

 low-pressure lighting circuits. The danger of a short or a 

 leak between high- and low-pressure circuits does not lie 

 in the transformer. This can be made absolutely safe ; but 

 the switches and leads to the transformer, and especially 

 the outside lines where there are miles of them, are a 

 source of danger. A broken wire or a branch of a tree 

 blown across two lines by the wind are possibilities from 

 which no excellence of workmanship can guard us. Some 

 means should therefore always be provided to cut off the 

 current automatically in the low-pressure circuit as soon 

 as its potential to earth exceeds a predetermined limit. 

 Such an instrument was perfected last year by Mr. Arcioni, 

 of Milan, and is now being gradually taken up on the 

 Continent. Last year I tested the Arcioni safety device 

 on the Milan system, making artificial leaks from the 

 6000-volt network to a local secondary lighting circuit, and 

 found the action absolutely trustworthy. 



The commercial development of electric power distribu- 

 tion on a large scale in this country by companies estab- 

 lished for this purpose may be said to have begun with 

 the present century. The public generally, and even some 

 engineers, are still under the impression that a country 

 of abundant water-power offers better opportunities for 

 electric power distribution than a country of cheap coal, 

 but that this is in reality not so is demonstrated by the 

 great development which power supply has reached in this 

 country. In the countr}' of waterfalls industries have to 

 be introduced in order to utilise the power made available 

 through electric transmission, whilst in the coal country 

 highly developed industries of different kinds are already 

 there. As regards capital outlay, the advantage lies gener- 

 ally with the thermal station, quite apart from the extra 

 cost of a steam reserve, which, for at least part of the 

 power, in many cases is unavoidable. If, then, we speak 

 of the cheap water-power of Swiss and Italian hydro- 

 electric works, we do not mean that those works can pro- 

 duce power more cheaply than English thermal stations, 

 but that they can produce it more cheaply than if they had 

 to use imported coal. 



Although in this country we have only little water-power, 

 the deficiency is made up by other sources of energy which 

 now mostly run to waste. Mr. C. H. Merz estimates that 

 within the area served by the North-east Coast Power 

 System the gas obtained as a by-product of the coke ovens 

 could be made to yield continuously 150,000 horse-power 

 if burned under boilers, and 250,000 horse-power if used in 

 internal-combustion engines. It is the merit of Mr. Merz 

 to have recognised the enormous commercial importance 

 of these sources of energy, and to have already made a 

 beginning with their utilisation by the establishment of 

 what he calls " waste heat stations." 



Electric Railways. 

 For main lines alternating current is unavoidable, and 

 the only question on which there may be still difference 

 of opinion is whether the current shall be three-phase or 

 single-phase. Electricians prefer the former, railway men 

 the latter mainly on account of the greater simplicity of 

 the overhead work. As the railway men are in reality the 



