246 BULLETIN OF THE 



than one-fifth of a mile) suspended several times across the large 

 room of the Academy, and placed in the galvanic circuit, the 

 same magnet sustained eight ounces : that is to say, the current 

 from the galvanic trough produced greater magnetic effect after 

 traversing this length of wire, than it did without it. 



Speculating on this remarkable, and at the time, paradoxical 

 result, Henry suggests in explanation, that " a current from a 

 trough possesses more 'projectile' force (to use Professor Hare's 

 expression,) and approximates somewhat in ' intensity ' to the 

 electricity from the common machine. May it not also be a fact 

 that the galvanic fluid in order to produce the greatest magnetic 

 effect should move with a smaller velocity, and that in passing 

 through one-fifth of a mile, its velocity is so retarded as to pro- 

 duce a greater magnetic action ? But be this as it may, the fact 

 that the magnetic action of a current from a trough is at least 

 not sensibly diminished by passing through a long wire, is 

 directly applicable to Mr. Barlow's project of forming an electro- 

 magnetic telegraph ; * and it is also of material consequence in 

 the construction of the galvanic coil From these experiments 

 it is evident that in forming the coil we may either use one very 

 long wire, or several shorter ones, as the circumstances may 

 require : in the first case, our galvanic combinations must consist 

 of a number of plates so as to give ' projectile' force ; in the 

 second, it must be formed of a single pair."f 



Here for the first time is presented to science the "intensity " 

 coil, — a spool of a single fine wire closely wound again and again 

 upon itself, — with its singular capabilities — not of power, but 

 (what was never before suspected nor imagined) of subtile ex- 

 citation from a distant source. Here for the first time is estab- 

 lished the important principle, that there must be a proportion 

 between the aggregate internal resistance of the battery, and the 

 whole external resistance of the conjunctive wire or conducting 

 circuit ; that for a " quantity" magnet of multiple coils (or their 

 equivalent a large wire of corresponding weight) a " quantity" 

 battery of surface, or a single galvanic element is required ; 

 while for an "intensity" magnet of extended continuous fine coil, 

 an " intensity " battery of many small' pairs is requisite :J with 

 the further discovery that the electro-motive force of the latter 

 form enables a very long conductor to be employed without seu- 



* Really Laplace's project; — not Barlow's. 



I Silliiuairs Am. Jour. Sci. Jan. 1831, vol. xix. pp. 403, 404. 



j " For circuits of small resistance, galvanometers of small resistance 

 must be used. For circuits of large resistance, galvanometers of large 

 resistance must also be used ; not that their resistance is any advantage, 

 but because we cannot have a galvanometer adapted to indicate very 

 small currents without having a very large number of turns in the coil, 

 and this involves necessarily a large resistance." Prof. F. Jenkin. Ehc- 

 tricity and Magnetism, 12mo. London, and N. Y. 1&73, chap. iv. sect. 8, 

 p. 89. 



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