388 



SCIENCE. 



[Vol. IV., No. 89. 



turned on; and gradually this stuff begins to boil. 

 You cannot take up this evaporated mass and tell 

 what will be its quality, until it has been evapo- 

 rated. Then, formulating these three things, you can 

 tell what temperature you want to use under this tar 

 in a boiling state. Then I have devised a process of 

 quite suddenly increasing the heat. This is done by 

 means of a double oven, one part being heated mod- 

 erately, and the other to quite a high degree of heat. 

 When the tar no longer boils, it is suddenly carbon- 

 ized. You then have to make your plate. I take 

 this substance and cut it. It is true, it is carbon, 

 and it is pretty hard, and there is difficulty in cutting 

 it: so I have devised another method to free myself 

 of the trouble of cutting the plates, and that is, to 

 prepare the plates of a proper size. This is a process 

 which I developed last winter. The method of man- 

 ufacturing which I have described is susceptible of 

 considerable uniformity. The character of these 

 plates is porous, and they are of considerable value 

 in a secondary battery. 



Plants, of course, is the great originator of second- 

 ary batteries of modern times. You can take a Plante 

 cell, and you can get forty per cent from it if it is 

 properly charged. A fair battery will return proper 

 work up to ninety per cent. It does not always do 

 it. It has to be carefully charged, and it will return, 

 under careful circumstances, ninety per cent. 



Mr. Frank J. Sprague. — Ninety per cent of the 

 form : is that what the speaker has told us ? I should 

 like to know what part of the circuit he uses. Does 

 it include the battery ? 



Mr. Koyle. — Including the battery, of course. 



Professor Forbes. — I merely wish to draw atten- 

 tion to this very point. The difficulty about manu- 

 facturing those plates is to get them of a proper 

 length. We should know exactly what we are deal- 

 ing with. The action is exactly, of course, as it has 

 been described. 



I want to show to you how the equations come out, 

 because they are very interesting in showing the 

 regularity of the secondary battery upon the bright- 

 ness of the lamps. 



DYNAMO 



b b b 



B 



Let JE 1 ] be the electromotive force of the dynamo, 

 J2x the resistance of it; E 2 the electromotive force of 

 the secondary battery, B 2 its resistance: E 3 we will 

 call the resistance of the electric lamps in the circuit. 

 That will represent the resistance of the lights which 

 come into play. This is given by a well-known law. 

 Perhaps that is the simplest way to take it. Let us 

 call the potential at the point A, e u and that at the 

 point B, e 2 . These are the two points where all the 



three circuits unite, and e x and e 2 are the potentials 

 at those points. Suppose that e x is greater than e 2 , 

 then the current circulating in each of these three 

 circuits can be put down immediately in terms of the 

 quantities which are there shown, involving the two 

 unknown quantities e l5 e 2 ; the current which is 

 circulating in the partial circuit of the dynamo 

 always taking the direction of the current as positive 

 when the current flows from e^ to e 2 . The current 



in this part of the circuit will be 



ei 



e 2 —E 



= C lt 



jBj being the resistance of this circuit. C 2 is obtained 

 from Cj by replacing E^ and i?, by E 2 and B 2 respec- 

 tively, and similarly for C 3 ( B z being the resistance of 

 the lamp-circuit). Finally, we have the equation that 

 the sum of all these equals zero; that is to say, the 

 total current which passes through any one point is 

 equal to zero, which is a well-known law. We have, 

 therefore, the sum of these currents (Cx + C 2 + C 3 ) 

 equal to zero. 



Now, this {e x ~e 2 ) is in reality the unknown quan- 

 tity. It is a difference of potentials. It is the single 

 unknown quantity of this equation. The result which 

 we arrive at on working out this equation is, that 



<?i — e 2 



_ E,B, + E,R l 



This is a very useful for- 



B 1 + B 2 



mula, and probably is well known, and has important 

 applications here. The application especially is to 

 finding the current which flows through the lamp- 

 circuit. The current C 3 , which is the current flow- 



. ^ u *v i • i + E,B, + E S B,. 



mg through the lamps, is equal to — |—p 



that is, the final value of the current which is pass- 

 ing through the resistance of the lamp-circuit. 



Now, here is a remarkable thing, — that, however 

 much the irregularity of the dynamo-machine may 

 be, the current is found to be very steady in the lamp- 

 circuit when we use the secondary battery in that 

 position. The reason of that is, the secondary bat- 

 teries which we are in the habit of using have an 

 extremely small resistance; and, whenever we use 

 secondary batteries, you will understand that we 

 use an infinitesimally small resistance, compared with 

 the resistance of the battery or the resistance of any 

 such lamp-circuit. 



If, then, we take the resistance of the secondary bat- 



E 2 

 tery to be zero, our equation simply gives us, <7 3 = ^-> 



with the other quantities cut out; that is to say, the 

 current is exactly the same as if this dynamo was not 

 working at all, and as if we had a battery of infinitesi- 

 mal resistance, with this electromotive force of E 2 

 working through these series of lamp-circuits. 



As I am here, I may as well make a few remarks 

 upon another point in connection with these second- 

 ary batteries. Of course, the great difficulty we 

 have had in the past times was, in the first place, 

 that we used thin plates, and they buckled. This has 

 been the most serious objection that we have had. 

 And it is very satisfactory to hear from Mr. Preece 

 that he found that he practically gets over the buc- 

 kling of the Faure cell and Volckmar battery. The 



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