350 HENRY AND THE TELEGEAPH. 



To this letter Henry replied as follows : 



Princeton, May 6, 1839. 



Dear Sir : Your favor of the 24tli ultimo came to Princeton during my absence, 

 wliich will account for the long delay of my answer. I am pleased to learn that you 

 fully sanction the loan which I obtained from Dr. Gale of your wire; and I shall be 

 happy if any of the results arc found to have a practical bearing on the electrical tele- 

 graph. It will give me ranch pleasure to see you in Princeton after this week ; my en- 

 gagements will not then interfere with our communications on the siibject of electric- 

 ity. I am acquainted with no fact which would lead me to suppose that the project 

 of the electro-magnetic telegraph is impracticable ; on the contrary, I believe that sci- 

 ence is now ripe for the ai^plication, and that there are no difficulties in the way, but 

 such as ingenuity and enterprise may obviate. But what form of the apparatus, or 

 what application of the power, will prove best, can I believe be only determined by 

 careful experiment. I can say however that so far as I am acquainted with the mi- 

 nutijB of your jilan, I see no practical difficulty in the way of its application for com- 

 paratively short distances ; * but if the length of the wire between the stations be 

 great, I think that some other modification will bo found necessary in order to develop 

 a sufficient power at the farther end of the line.t I shall however be happy to con- 

 verse freely vrith you on these points when we meet. In the meantime, I remain, 



" With much respect, 



"Yours, &c., 



"Joseph Hknry." 



A short time after this. Professor Morse visited Henry at Princeton; and during this 

 first personal interview in May, 1839, received satisfactory answers to various ques- 

 tions presented. Among them, Henry stated that he had no reason to doubt that 

 magnetism could be induced in soft iron "at the distance of a hundred miles or more 

 by a single impulse or from a single battery:" (a striking expression of faith in his 

 own "intensity" magnet:) also, that with a given battery, circuit, and electro-magnet 

 at a distance, the inclusion of intermediate electro-magnets at way-stations would 

 not sensibly reduce the magnetic i)Ower at the several points. On the subject of the 

 differences between "quantity" and "intensity" magnets. Professor Morse was still 

 greatly in the dark, and he asked the question, "Is it quantity or intensity which has 

 most effect in inducing magnetism in soft iron ? " Honry fully explained to him that 

 for producing the greatest magnetic effects, a "quantity" magnet and battery, with 

 short and free circuit, were required ; but that for a long circuit (required for magnety- 

 izing at a distance), an "intensity" magnet and battery were indispensable. t 



* [It must be borne in mind that this was a year and a half after the writer had been 

 informed by Dr. Gale of his successful experiment through ten miles of wire. By 

 "comparatively short distances," therefore, he must evidently have intended distances 

 less than those separating our princijial cities.] 



t[Tlie j)ecaliar form of expression hero used, suggesting the probable occasion for 

 another modification "in order to develop a sufficient power — at the farther end of tlie 

 line," points directly to his own contrivance (exhibited before his classes, four years 

 previously) of a supplemental "quantity" magnet and battery at the distant station. 

 Henry had no doubt of being able to magnetize iron at a distance of several hundred 

 miles, and hence evidently did not contemplate dividing the lino into a "relay" of 

 circuits, as devised by Professor Morse. His language that a re-enforcement may be 

 necessary "if the lenfjthof the wire between the stations be great," plainly shows this. 

 But he anticipated that the attractive power developed would be feeble ; while he de- 

 clared his confidence that there Avere no scientific "difficulties in the way." Surprise 

 has been expressed that Henry did not frankly "give Professor Morse the benefit of this 

 solution of the suggested difficulty, if it were then in his mind. But is not the subsequent 

 reticence of Professor Morse, on the expedient of a "relay" (invented by him two 

 years pi'eviously, as alleged), much more surprising ? Henry referred to the enfeeble- 

 ment on a longline, as a merely " practical difficulty" easily "obviated by ingenuity."] 



t Of the communications made on the occasion of this very interesting and impor- 

 tant interview, occux)ying an "afternoon and evening," we have unfortunately only 

 the result furnished by Professor Morse's very meager statement. (Prime's Life of 

 Morse, chap, x, i). 422.) That Henry, in explaining the differing functions of the two 



