20 



NATURE 



\_May 5, i\ 



visible and small in tlie operations of men who work at pure 

 chemistry and physics, and it is especially true of laboratory 

 work in electricity that every day a man sees new lines of 

 research opening up before him which his resources do not 

 allow him to follow up. 



In the ai'plied science of electricity certain fixed laws tell us 

 much about the future which is not generally known ; and it is 

 first necessary to become acquainted with these laws if we would 

 speak of this future. By numerous experiments the lecturer 

 showed that electricians are dealing with measurable thing?, and 

 he gave in wall-sheets such information as seemed sufficient to 

 give exact ideas in this matter to a popular audience. These 

 wall -sheets had also been put in a printed form and circulated 

 among the audience. The following is an example : — 



Wall-sheet II. — Electrical Magnitudes 

 (some rather approximate) 

 Resistance of 



One yard of copper wire, one-eighth of 



an inch diameter 0'002 ohm. 



One mile ordinary iron telegraph wire lo to 20 ohms. 



Some of our selenium cells 40 to 1,000,000 



A good telegraph insulator 4,000,000,000,000 



Electrotnotive force of 



A pair of copper-iron junctions at a Volts, 



difference of temperature of 1° Fahr. = o'00O,0I 



Contact of zir.c and copper = 0'75 



One Daniell's cell = I'l 



Mr. Latimer Clark's standard cell ... = l"45 



One of Dr. De La Rue's batteries ... = 11,000 

 Lightning flashes probably many millions of volts. 



Current measured by us in some experiments : — 



Using electrometer = almost infinitely small 



currents. 



Weber. 



Using delicate galvanometer =o'ooo,coo,ooo,040 



Current received from Atlantic cable, 

 when twenty-five words per minute 

 are being sent =O"00O,00i 



Current in ordinary land telegraph 



lines =0*003 



Current from dynamo machine ... =5 to 100 Webers 



In any circuit, current in webers ... = electromotive force 

 in volts -T- resistance in ohms. 



Wall-sheet III. — Rate of Production of Heat calcu- 

 lated IN THE shape of HORSE-POWER 

 In the whole of a circuit = current in webers X electromotive 



force in volts -=- 746. 

 In any part of circuit = current in webers X difference of poten- 

 tial at the two ends of the part of the circuit in question -^ 

 746. 

 Or, = square of current in webers X resistance of the part in 

 ohms -^ 746. 



The distinction which mu-it be made between electricity and 

 electrical energy was dwelt upon. A miller does not merely 

 speak of the quantity of water in his mill-dam ; he has also to 

 consider the height through which it can fall. A weight of one 

 thousand pounds falling through a distance of one inch repre- 

 sents the same energy, that is, gives out the same amount of 

 work in falling as one pound through one thou-and inches. A 

 mere statement then of the quantity of electricity given out by a 

 machine is insufficient ; it is also necessary to state what is the 

 height or difference of potential through which it is falling. The 

 quantity of electricity in a thunder-cloud is comparatively small, 

 but the difference of potential through -which this quantity 

 passes when discharge occurs is exceedingly great. So it is with 

 the two factors of the electrical energy developed by this glass 

 machine. The quantity of electricity obtainable from this 

 machine is comparatively small, but it is like a small quantity of 

 water at an exceedingly great height, whereas in all these other 

 machines we have, in the analogy of the miller, a very great 

 quantity of water and a very small difference of level. I put this 

 water analogy before you because you have all more or less 

 exact notions about water, and because, within certain limits, 

 the analogy is a very true one. I have traced it more fully in 

 the wall-sheet. Of this and the other wall-sheets each of you 

 possesses printed copies. 



Wall-sheet I. 



IVe Want to Use Water. 



I. Steam-pump burns coal 

 and lifts water t j a higher level. 



dif- 



2. Energy available 

 amount of water lifted > 

 ference of level. 



3. If we let all the water 

 flow away through channel to 

 lower level without doing work, 

 its energy is all converted into 

 heat because of frictional resist- 

 ance of pipe or channel. 



4. If we let water work a 

 hoist as well as flow through 

 channels, less water flows than 

 before, less power is wasted in 

 friction. 



5. However long and narrow 

 may be the channels, water 

 may be brought from any dis- 

 tance, however great, to give 

 out almost all its original energy 

 to a hoist. This requires a 

 great head and small quantity 

 of water. 



We Want to Use Electricity. 



1. Generator burns zinc, or 

 uses mechanical power, and 

 lifts electricity to a higher level 

 or potential. 



2. Energy available is, 

 amount of electricity x differ- 

 ence of potential. 



3. If we let all the electricity 

 flow through a wire from one 

 screw of our generator to the 

 other without doing work, all 

 the electrical energy is con- 

 verted into heat because of re- 

 sistance of wire. 



4. If we let our electricity 

 work a machine as well as flow 

 through wire?, less flows than 

 before, less power is wasted 

 through the resistance of the 

 wire. 



5. However long and thin 

 the wires may be, electricity 

 may be brought from any dis- 

 tance, however great, to give 

 out almost all its original energy 

 to a machine. This requires a 

 great difference of potentials 

 and a small current. 



After showing, by passing currents from two large Gramme 

 machines through certain resistances and lamps, that electrical 

 energy may be sent to a distant place and there converted into 

 heat and light, the methods taken at the City and Guilds of 

 London Institute for simultaneously measuring mechanical work, 

 currents of electricity, resistance, the candle-power of electric 

 lamps, &c., were described, the dynamometers, photometers, 

 &c., being exhibited, as well as diagrams showing their con- 

 struction. Actual measurements were made of the strengths of 

 currents and the candle-power of an electric light. Many of the 

 contrivances in use were invented by the lectm-er and his friend 

 Prof. Ayrton. 



The transmission of mechanical power to a distance through 

 the agency of electricity was illustrated by a number of experi- 

 ments ; the driving of a lathe and other machines, and proof 

 that the motor which gives out power at the distant place pro- 

 duces a back electromotive force opposed to that of the generator. 



"Now. what do these examples show you? They show that 

 if I have a steam-engine in my back yard I can transmit power 

 to various machines in my house, and if you were to measure the 

 power given to these machines you would find it to be less than half 

 of what the engine driving the outside electrical machine gives to 

 it. Further, when we wanted to think of the heating of buildings 

 and the boiling of water, it was all veiy well to sj eak of the 

 conversion of electrical energy into heat, but now we find that 

 not only do the two electrical machines get heated and give out 

 heat, but heat is given out by our connecting-wires. We have 

 then to consider our most important question. Electrical energy 

 can be transmitted to a distance, and even to many thousands of 

 miles, but can it be transformed at the distant place into 

 mechanical or any other required form of energy, nearly equal 

 in amount to what was supplied ? Unfortunately I must say that 

 hitherto the practical answer made to us by existing machines is 

 ' No ' ; there is always a great waste due to the heat spoken of 

 above. But fortunately we have faitli in the measurements, of 

 which I have already spoken, in the facts given us by Joule's 

 experiments and formulated in ways we can understand. And 

 these facts tell us that in electric machines of the future, and in 

 their connecting-wires, there will be little he.ating, and therefore 

 little loss. We shall, I believe, at no distant date, have great 

 cenlr.\l stations, possibly situated at the bottom of coal-pits, 

 where enormous steam-engines will drive enormous electric 

 machines. We shall have wires laid along eveiy street, tapped 

 into every house, as gas-pipes are at present ; we shall have the 

 quantity of electricity used in each house registered, as gas is at 

 ]3resent, and it w ill be passed through little electric machines to 

 drive machinery, to produce ventilation, to replace .stoves and 

 fires, to work apple-parers, and mangles, and barbers' hrushe?. 



