September 19, 1S84] 



SCIENCE, 



297 



the ship, making connection with the sea at a con- 

 siderable distance from the vessel; and suppose the 

 current be rapidly made and broken by an inter- 

 rupter : then the observer on a second vessel provided 

 with similar terminal conductors to the first, but 

 having a telephone instead of a dynamo, will be able 

 to detect the presence of the other vessel even at a 

 considerable distance ; and by suitable modifications 

 the direction of the other vessel may be found. This 

 conception Professor Bell has actually tried on the 

 Potomac River with two small boats, and found that 

 at a mile and a quarter, the farthest distance experi- 

 mented upon, the sound due to the action of the 

 interrupter in one boat was distinctly audible in the 

 other. The experiment did not succeed quite so well 

 in salt water. 



Professor Trowbridge then mentioned a method 

 which he had suggested some years ago for telegraph- 

 ing across the ocean without a cable; the method 

 having been suggested more for its interest, than 

 with any idea of its ever being put in practice. A 

 conductor is supposed to be laid from Labrador to 

 Patagonia, ending in the ocean at those points, and 

 passing through New York, where a dynamo-machine 

 is supposed to be included in the circuit. In Europe 

 a line is to extend from the north of Scotland to the 

 south of Spain, making connections with the ocean 

 at those points: and in this circuit is to be included a 

 telephone. Then any change in the strength of the 

 current in the American line would produce a corre- 

 sponding change in current in the European line; 

 and thus signals could be transmitted. Mr. Preece, 

 of the English postal telegraph, then gave an account 

 of how such a system had actually been put into 

 practice in telegraphing between the Isle of Wight 

 and Southampton during a suspension in the action 

 of the regular cable communication. The instru- 

 ments used were a telephone in one circuit, and in the 

 other about twenty-five Leclanche cells and an inter- 

 rupter. The sound could then be heard distinctly; 

 and so communication was kept up until the cable 

 was again in working-order. Of the two lines used 

 in this case, one extended from the sea at the end of 

 the island near Hurst castle, through the length of the 

 island, and entered the sea again at Rye; while the 

 line on the mainland ran from Hurst castle, where 

 it was connected with the sea, through Southampton 

 to Portsmouth, where it again entered the sea. The 

 distance between the two terminals at Hurst castle 

 was about one mile, while that between the termi- 

 nals at Portsmouth and Rye amounted to six miles. 



A few years ago Mr. E. H. Hall, then a student at 

 the Johns Hopkins university, taking a thin strip of 

 gold-leaf through which a current of electricity was 

 passing, and joining the two terminals of a very 

 sensitive galvanometer to two points in the gold-leaf, 

 one on one edge, and the other on the other, choos- 

 ing the points so exactly opposite that there was no 

 current through the galvanometer, found that on 

 placing the poles of a powerful electro-magnet, one 

 above and the other below the strip of gold-leaf, he 

 obtained a current through the galvanometer, thus 

 indicating that there was a change in the electric 



potential, due to the action of the magnet. Mr. Hall 

 explains this change by supposing the rotation of 

 the equipotential lines in the conductor about the 

 lines of magnetic force. This explanation has been 

 brought into question by Mr. Shelford Bidwell, who 

 attempts to explain the action thus : The magnetic 

 force acting on the conductor carrying the current 

 would cause the conductor to be moved sideways, 

 were it free to move ; but, since it is held by clamps 

 at the ends, the magnetic force acting upon it brings 

 it into a state of strain, one edge being compressed 

 and the other stretched; and Mr. Bidwell supposes 

 the whole Hall effect to be due to thermal actions 

 taking place in consequence of this unsymmetrical 

 state of strain. Professor Hall, who is now at Har- 

 vard, has made some careful experiments to test this 

 explanation of Mr. Bidwell. He used not only gold- 

 leaf, but strips of steel, tinfoil, and other metals, and 

 clamped them sometimes at both ends, sometimes in 

 the middle, and sometimes only at one end; and in 

 all cases the ^action was the same, with the same 

 metal, irrespective of the manner of clamping. This 

 was strong evidence against Mr. Bidwell's position. 

 Sir William Thomson suggested, as a further test, to 

 bring about the state of strain, which Mr. Bidwell 

 supposes to be the primary cause of the action, by 

 purely mechanical means, bringing pressure to bear 

 on one side or the other, and seeing whether the 

 action obtained is at all commensurate with the ac- 

 tion found by Mr. Hall. 



Professor Hall then discussed an experiment by 

 which Mr. Bidwell had obtained a reversal of the 

 effect; and showed that the reversal was only appar- 

 ent, and that when carefully examined the results 

 of Mr. Bidwell's experiment were best satisfied by 

 the theory of the rotation of the equipotential sur- 

 faces about the lines of magnetic force. Sir William 

 Thomson spoke of the discovery of Mr. Hall as being 

 the most important made since the time of Faraday. 

 He favored Mr. Hall's explanation; though he con- 

 siders Mr. Bidwell's suggestion as very important, 

 and thinks that it will very likely be found that both 

 the Hall effect and thermal effects have a common 

 cause, rather than that one is to be taken to explain 

 the other. He showed also that the mathematical 

 examination of the subject indicates three relations 

 to be investigated, — first, the relation of thermal 

 force to the surfaces of equal rate of variation of 

 temperature ; second, the relation of electric current 

 to the equipotential surfaces; third, the relation of 

 the thermal flow to isothermal surfaces. The second 

 of these is that investigated by Mr. Hall, who has 

 found that under the conditions mentioned the lines 

 of flow are not perpendicular to the equipotential 

 surfaces. There remains, therefore, 'work for two 

 more Halls,' in either proving or disproving the ex- 

 istence of the analogous actions in these other two 

 cases. Sir William Thomson also suggested the fol- 

 lowing exceedingly interesting mechanical illustra- 

 tion or analogue of Hall's effect. Let us be living 

 upon a table which rotates uniformly forever. A 

 narrow circular canal is upon this table, concentric 

 with the axis of rotation of the table, and nearly 



