286 



SCIENCE. 



[Vol. II., No. 30, 



as badly insulated as it well could be. I have also 

 used it on tlie same line under the most favorable 

 conditions for insulation, and could not really per- 

 ceive nuidi difference. It seemed to be as loud at 

 one time as at another. 



Pres. II. A. KowL.'VND. — Of course this is on an 

 entirely different principle from our telephone. What 

 interested me considerably was the fact, that one 

 conld hear better when the plates were charged. 

 Tlie explanatiiin theorelically is very simple, and it 

 is the saints as that the Tliompson electrometer is 

 wore sensitive when the jar is charged than when it 

 is not charged; tlie reason being, that the attraction 

 is proportionate to the square of the difference of 

 the potentials, rather than the simple difference of the 

 potentials. Tlierefure a small diifereuce in the quan- 

 tity, when it is large, prodvices a greater effect than 

 •when it is small. So ihe explanation is exactly the 

 same as that the Thompson electrometer is more 

 sensitive when you have the jar charged than wh^n 

 you do not. So. the higher the charge one would get, 

 the more sensitive the instrument would be. I was 

 especially interested in it, because it was on such an 

 entirely different principle from the Bell instrument. 

 I don't wish to say any thing about patent laws or 

 decisions on this subji'ct, for they have nothing to do 

 ■with this; but, scientifically, this is an entiiely differ- 

 ent instrument from the Bell instrument, and I am 

 especially interested on that account. 



Prof. T. C. Mendknhai.l. — I profess not to have 

 quite uiulerstood the statement made by Professor 

 Dolbear. I should like to hear your own (the presi- 

 dent's) opiniim with regard to that charge which 

 remains in spite of the fact that the two poles of the 

 condenser are connected by conductors. I may have 

 misunilerstood the statement; but if that is correct, I 

 should like to know whether that can be explained 

 or not. 



President Rowland. —Well, I suppose we all know 

 how retentive an electroscope is of a charge. I sup- 

 pose tlie idea is very similar in this case. I do not 

 suppose the plates have a difference of potential. If 

 you should leave them for a moment, I should suppose 

 they woidd soon have a little return charge. If the 

 two plates of the condenser were together, tbey would 

 have the same potential. I understood it as merely 

 a return charge. I do not know how Professor Dol- 

 bear understands it. 



Professor Doi.BEAR. — The instrument itself is a 

 most delicate electrometer when tested in this way; 

 and when it is charged and really in good working 

 order, the gentlest tap uptm the instrument serves to 

 show that it is in good working order, for one can 

 apply the instrument to his e.arand hear himself talk. 

 This is the case, even when the two plates of the con- 

 denser are connected with each other through the 

 induction coil; and .=0, although they may have been 

 there for hours, or even for days, — the difference be- 

 tween an instrument that has not been used and one 

 that has been charged is very appreciable. 



President Rowland. — I suppose in that case it 

 would be simply from the charge of the varnished 

 surface ? 



Professor Dolbear. — Yes: I think they retain 

 their charge for a much longer time if (lie surface is 

 varnished. I do think there is a difference between 

 the behavior of this and the charged cable. If a 

 cable be charged for half an hour by battery, it will 

 require half an hour to run out again, but it will be 

 at that time quite discharged. But that is not the 

 case with this instrument. 



President Rowland. ^ I should suppose it was the 

 charge in the varnished surface. 



Prof. W. A. Anthony. — Professor Dolbear did 

 not say any thing about one advantage that this tele- 

 phone has over the other, that struck me when I read 

 the descriptions of it earlier, — that, in conseqtience 

 of using this electricity at such a high potential, the 

 ordinary telegraph-lines or other instruments would 

 have very little effect upon it: therefore the tele- 

 phone is very free from induction. 



Professor Dolbear. — My experience has been in 

 accordance with that theory. Electro-motive force 

 from induction from telegraph-lines is ordinarily 

 tolerably small, although there may be at times con- 

 siderable strength of current. But, the electro-motive 

 force being so strong in my circuit, it follows that the 

 action of such induced currents is very slight, and 

 does not interfere with work. 



Prof. C. A. Young. —I would like to inquire 

 whether you have tried any experiments in putting 

 the end of the wire to the ear to illustrate the sensi- 

 tiveness of the ear ? 



Professor Dolbear. — Yes: I have heard simply 

 by putting the end of an insulated wire to my ear, 

 and listening. I consider the instrument as simply 

 the enlarged terminal of a wire, and that you are 

 actually listening at the end of a wire. 



Mr. E. Gkay. — I have made a good many experi- 

 ments in another line, which I may state briefly, 

 which may throw some light upon this, and yet I think 

 it is very well understood. You remember, some 

 of you, reading of such experiments made in 1S74, 

 relating to the reproducing of music on a plate by 

 simply rapping with the finger or with some animal 

 tissue. Now, I made this experiment, which seems 

 to prove to my mind that the operation is as Profes- 

 sor Dolbear has explained it. I set my revolving 

 disc, which was a simple disc of zinc, revolving at a 

 steady rate, giving it a pressure with the fingers. 

 Then I had fifty cells of battery set up, as much 

 as I could bear, passing through them, and had 

 some one close the circuit with a Morse key. At the 

 Siime time the key was closed, my finger would be 

 jerked forward in the direction of the rotation of the 

 disc; and it would remain in that forward condition, 

 showing an increase of friction, until the key would 

 be opened, and then it would drop back; showing 

 that from some cause there was an increase of friction, 

 either due to molecular disturbance, or, what is prob- 

 ably the case, to attraction between the finger and 

 the plate. It is necessary, to produce this experi- 

 ment, that the cuticle be perfectly dry. You must 

 rub it a long time, and have it perfectly polished ; and 

 then the cuticle becomes a dielectric, and the body is 

 charged with one kind of electricity, and the wire or 



