Jan. 20, 1887] 



NA TV RE 



279 



contact instantaneously after it is made ; otherwise all but 

 one of the indications of several meters, whose times of 

 contact with the tooth on the disk overlapped, would fail 

 to be registered at the central station, and should the 

 stoppage of any one engine in the district happen to 

 occur while this tooth of its meter was in contact the 

 whole registering apparatus would cease to act for an 

 indefinite time. 



The contact-breaker is shown in Fig. 3, at the left-hand 

 side. The momentary current caused on making contact 

 magnetises an electro-magnet, which, by attracting its 

 armature, draws the contact-maker (which is mounted on 

 a piece of watch-spring) past the tooth into such a position 

 that it catches behind a small plate of insulating material 

 at the back of the tooth, which prevents it springing again 

 into contact with the latter when the armature of the 

 magnet is released. 



Fig. 4 explains the calculation of the thermodynamic 

 efficiency of this mode of transmission of power. It is 

 drawn for unit volume of atmospheric air drawn into the 

 air-pumps. The pressures are reckoned in atmospheres. 

 ."^ K c D E is the indicator-diagram showing the work done 

 by the compressor-pump. The compression-curve c d is 

 taken according to the law/ sc 7/-' = because it seems pro- 

 bable that this index may be reached with the efficient 

 water-cooling system adopted. The suction-line A B is 



taken .,'. atmosphere below atmospheric pressure. The 

 point F is taken on the same isothermal as C ; thus D F is 

 the loss of volume consequent on the air cooling in the 

 pipes down to atmospheric temperature. The dia- 

 gram E F G H is the indicator-diagram for an engine driven 

 by the air without loss of initial pressure below the com- 

 pressor pressure, without clearance, without expansion, 

 and with a back pressure -^^ atmosphere above atmo- 

 spheric pressure. The same back pressure is used for all 

 the other engine diagrams. The diagrams e f i k h, 

 E F L M H, and E F N H are diagrams for engines with 

 similar conditions, and with ratios of expansion i\, 2, and 

 24 ; that is, with cuts off f, i, and -, the last being that 

 that brings the final pressure down to I ^^j atmosphere. 

 The expansion-curve filn is taken as adiabatic. If 

 i atmosphere be lost in frictional and viscous resistance 

 to flow through the pipes, by obstructions at bends, passage 

 through meter, &c., or by sudden change of section of 

 pipe, then the admission line is lowered to P Q. The 

 effect of clearance is to cut off a part of the diagram by a 

 vertical line at the left-hand end. This vertical line is not 

 drawn in the diagram, because its position varies with the 

 grade of expansion employed In calculating the following 

 results the clearance has in each case been taken as ^'o the 

 volume of the cylinder. The area of the compressor- 

 diagram is I '6, and the efficiency is in each case obtained 



F:g. 



by dividing the engine-diagram area by i'6, and multi- 

 plying this quotient by /g. This ^-'j is the ratio between 

 the compressor-diagram and that of the central station 

 engine which drives it, the mechanical inefficiency of this 

 central plant being taken as y\j. The results are most 

 clearly shown in tabular form. 



Table of Efficiencies of Transmission of Poiuer by Air 

 compressed to 45 pounds per square inch 



Efficiency 

 Ratio of expan<.ion = i iV 2 9?, 



No loss of initial pressure 1 

 No clearance > 



Back pressure I'l atmos. 

 Initial pressure 3 '8 atmos. 

 No clearance 

 Back pressure I 'I atmos. 

 No loss of initial pressure 

 Clearance ^'j vol. of cyl. 

 Back 'pressure I • I atmos. 

 Initial pressure y% atmos 

 Clearance jV vol. of cyl. 

 Backpressure i-i atmos. 



■45 



■58 



•67 



•36 



■47 



•54 



•57 



The last two sections of this table comprise the limits 

 of practicable results. The highest efficiency shown is 60 

 per cent. This could only be obtained by avoiding abso- 

 lutely all loss of pressure between compressors and air- 



engine. This can hardly be accomplished even if the engine 

 be situated close to the central works. It need hardly be 

 pointed out that the expansion will not usually be carried 

 so far as to bring the working pressure to near equality 

 with the back pressure ; in fact, to do so is decidedly very 

 bad practice, and does not lead to economy in the brake- 

 power, especially when depreciation and interest on first 

 cost of the engine is taken into account. With good 

 management, from 30 to 50 per cent, efficiency may be 

 expected. 



In a paper read by Mr. Sturgeon before the British 

 Association last summer, he gives a table of calculated 

 efliciencies ranging from -32 to '84. These calculations 

 include allowances of 2 per cent, for valve-resistance and 

 leakage past compressor-piston ; 13 per cent, for leakage, 

 friction, and wire-drawing in the pipes ; and 8 per cent, 

 for clearance and back pressure in the consumer's engine. 

 Except the last, these allowances are much more liberal 

 than those that have been made in calculating the above 

 table. On the same basis as ours have been made, Mr. 

 Sturgeon's calculations would have given considerably 

 higher figures than the above '32 to '84. But the higher 

 figures in Mr. Sturgeon's table are obtained by supposing 

 that the consumer heats the air by a gas-stove, before 

 passing it into his engine, up to temperatures from 212° F. 

 to 320" F. How the resulting figures can be in any sense 



