256 H. S. Car hart — Surface Transmission 



Art. XXIV. — On Surface Transmission of Electrical Dis- 

 charges ;* by H. S. Carhart. 



The Eeport of the Commissioner of Patent? for 1859, volume 

 " Agriculture," contains an important but little known contri- 

 bution by Professor Joseph Henry on " Atmospheric Elec- 

 tricity." Under the head of " Electricity in Motion " occurs 

 this passage : — " If the discharge be not very large in propor- 

 tion to the size of the conductor, it will principally be trans- 

 mitted at the surface. If the charge be very large, and the 

 conductor small, it will probably pervade the whole capacity, 

 and, as we have seen, will convert into an impalpable powder 

 or vapor the solid particles." Of the latter statement abundant 

 demonstration has recently come under the writer's notice To 

 settle the former point the author of the paper alluded to 

 instituted a series of experiments, only one of which is described. 

 It is fair to infer that this one was deemed the most conclusive. 

 The arrangement in his own words was as follows : " C D is a 



copper wire of the size usually employed for ringing 



door-bells, passing through the axis of an iron tube, or a piece 

 of gas-pipe, about three feet long. The middle of this wire was 

 surrounded with silk, and. coiled into a magnetizing spiral, into 

 which a large sewing needle was inserted. The wire was sup- 

 ported in the middle of the tube by passing it through a cork 

 at each end, covered with tin-foil, so as to form a good metallic 

 connection between the copper and the iron. F and Y are two 

 other magnetizing spirals of iron wire, on opposite sides of the 

 tube, the ends soldered to the iron. When these two spirals 

 were also furnished with needles and a discharge from a Leyden 

 jar sent through the apparatus, as if to pass along the wire, the 

 needle inside of the iron tube was found, to exhibit no signs of 

 magnetism, while those on the outside presented strong polarity. 

 This result conclusively shows that, notwithstanding the interior 

 copper wire of this compound conductor was composed of a 

 material which offered less resistance to the passage of the 

 charge than the iron of which the outer portion was formed, 

 yet when it arrived at the tin-foil covering of the cork, it di- 

 verged to the surface of the tube, and. still further diverged 

 into the iron wire forming the outer spirals." 



Several years since I repeated this experiment apparently 

 with the result described above. Doubt was cast on the demon- 

 stration, however, by the discovery in one trial that the inner 

 needle was slightly magnetized. 



*Read before the American Association for the Advancement of Science, 

 August 21, 1885. 



