J tine 14, 1883] 



NA TURE 



163, 



position continuously, to keep an exact record of the correspond- 

 ing rates at which the water is passing, every minute, or, better, 

 every second, and to add up all the values obtained This 

 would of course be a very troublesome process. There is an- 

 other kind of instrument which may be used to measure the 

 flow of the water: a paddle-wheel or screw. When the water 

 is flowing rapidly the wheel will turn rapidly; when slowly, 

 the wheel will turn slowly ; and when the water flows the other 

 way the wheel will turn the other way, so that if we observe 

 how fast the wheel is turning we can tell how fast the water is 

 flowing. If now we wish to know how much water altogether 

 has passed through the trough, the number of turns of the 

 wheel, which may be shown by a counter, will at once tell us. 

 There are therefore in the case of water two kinds of instru- 

 ments, one which measures at a time, and the other during a 

 time. The term meter should be confined to instruments of the 

 second class only. 



As with water so with electricity, there are two kinds of mea- 

 suring instruments, one, of which the galvanometer may l>e 

 taken as a type, which shows by th-i position of a magnet how 

 strong a current of electricity is at a time, and the other, which 

 shows how much electricity has | assed during any time. Of 

 the fir t, which are well understood, I shall say nothing ; the 

 second, the new electric meters and the corresponding meters 

 for power, are what I have to speak of to-night. 



It is hardly necessary for me to mention the object of making 

 electric meters. Every one who has had to pay his gas bill once 

 a quarter probably quite appreciates what the electric meters are 

 going to do, and why they are at the present time at'racting si 

 much attention. So soon as you have electricity laid on in your 

 hiu-es, as gas and water are laid on now, so soon will a meter 

 of some sort be necessary in order that the companies which 

 supply the electricity may be able to make out their quarterly 

 bills, and refer complaining customers to the faithful indications 

 of their extravagance in the mysterious cupboard in which the 

 meter is place I. 



The urgent necessity for a good meter has called such a host 

 of inventors into the field that a complete account of their 

 labours is more than any one could hope to give in an hour. 

 Since I am one of this host I hardly like to pick out those in- 

 ventions which I consider uf value. I cannot describe all, I cannot 

 act as a judge and say these only are worthy of your attention, 

 and I do not think 1 should be acting fairly if I were to describe 

 my own instru nents only and ignore those of every one else. 

 The »nly way I see out of the difficulty is to speak more par- 

 ticularly about my own work in this direction, and to speak 

 generally on the work of others. 



I must now ask you to give your attention for a few minutes 

 to a little abstract geometry. We may represent any changing 

 quantity, as for instance the strength of an electrical current, by 

 a crooked line. For this purpose we must draw a straight line 

 to represent time, and make the distance of each point of the 

 crooked line above the straight line a measure of the strength 

 of the current at the corresponding time. The size of the figure 

 will then measure the quantity of electricity that has passed, 

 for the stronger the current is the taller the figure will be, 

 and the longer it la ts the longer the figure will be, either 

 cause makes both the quantity of electricity and the size of the 

 figure greater and in the same proportion ; so the one is a mea- 

 sure of the other. Now it is not an easy thing to measure the 

 size of a figure : the distance round it tells nothing ; there is, 

 however, a geometrical method by which its size may be found 

 Draw another line, with a great steepness where the figure is 

 tall, and with a less steepness where the height is less, and with 

 no steepness or horiz >ntal where the figure has no height. If 

 this is done accurately, the height to which the new line reaches 

 will measure the size of the figure first drawn ; for the taller the 

 figure is, the steeper the hill will be ; the longer the figure, the 

 lo.'ger the hill ; either cause makes both the size of the figure 

 and the height of the hill greater, and in the same proportion; 

 so the one is a measure of the other ; and so, moreover, is the 

 height of the hill, which can be measni ed by a scale, a measure 

 of the quantity of electricity that has passed. 



The first instrument that I made, which I have called a "cart " 

 integrator, is a machine which, if the lower figure is traced out, 

 will describe the upper. I will trace a circle ; the instrument 

 follows the curious bracket-shaped line that I have already made 

 sufficiently black to be seen at a distance, the height of the new 

 line measures the size of the circle, the instrument has squired 

 the circle. This machine is a thing of mainly theoretical interest, 



my only object in show ing it is to explain the means by which I 

 have developed a practical and automatic instrument of which I 

 shall speak presently. The guiding principle in the cart inte- 

 grator is a little three-wheeled cart, whose front wheel is con- 

 trolled by the machine. This, of course, is invisible at a distance, 

 and therefore I have here a large front wheel alone. On 

 moving this along the table, any twisting of its direction instantly 

 cans s it to deviate from its straight path ; now suppose I do 

 not let it dev ate, but compel it to go straight, then at once a 

 great strain is put upon the table which is urged the other way. 

 If the table can move it w ill instantly do so. A table on rollers 

 is inconvenient as an instrument, let us therefore roll it round 

 into a roller, then cm moving the wheel along it the roller will 

 tin 11, and the amount by which it turns will correspond to the 

 height of the second figure drawn by the cart integrator. If, 

 therefore, the wheel is inclined by a magnet under the influence 



Fig. 1. 



of an electric current, or by any other cause, the whole amount 

 of which we wish to know, then the number of turns of the 

 roller will tell us this amount ; or to go back to our water 

 an logy, if we had the weighted board to show current strength, 

 and had not the paddle-wheel to show total quantity, we might 

 use the board to incline a disk in contact with a roller, a'ld then 

 drag the roller steadily along by clockwork. The number of 

 turns of the roller would give the quantity of water. Instru- 

 ments that will thus add up continuously indications at a time, 

 and so find amounts during a time, are called integrators. 



The most important application that I have made at present 

 of the integrator described is what I hive called an engine- 

 power meter. The instrument is on the table, but as it is far too 

 small to be seen at a distance, 1 have arranged a large model to 

 illustrate its action. The object of this machine is to measure 

 how much work an engine has done during any time, and show 



ft 



Fig. 2. Fie. 3. 



the result on a dial, so that a workman may read it off at once 

 without having to make any calculations. 



Before I can explain h:>w work is measured, perhaps I had 

 better say a few words about the meaning of the word "work." 

 Work is done when pressure overcomes resistance, producing 

 motion. Neither motion nor pressure alone is work. The two 

 factors, pressure and motion, must occur together. The work 

 done is found by multiplying the pressure by the distance moved. 

 In an engine, steam pushes the piston first one way, then the 

 other, overcomes resistance, and does work. To find this, we 

 must multiply the pressure by the motion at every instant, and 

 add all the products together. This is what, the engine-power 

 meter does, and it shows the continuously growing re-ult on a 

 dial. When the piston moves it drags the cylinder along, where 

 the steam presses the wheel is inclined. Neither action alone 

 causes the cylinder to turn, but when they occur together the 

 cylinder turns, and the number of turns registered on a dial 

 shows with mathematical accuracy hiw much work has been 



done. 



In the steam-engine work is done in an alternating manner, 



