596 



NA TURE 



[Oct. 1 6, il 



32. All electrostatic determinations of contact force are really 

 determinations of the sum of at least three such forces, none of 

 which are knowable separately by this means. 



33. The only direct way of investigating contact force is by 

 the Peltier effect or its analogues. [Maxwell.] 



34. Zinc and copper in contact are oppositely charged, but 

 are not at very different potentials : they were at different 

 potentials before contact, but the contact has nearly equalised 

 them. 



[Certain portions of these statements which may appear wildly 

 hypothetical, such as 13, are to be justified by figures. The 

 justification is not complete, for lead and iron are untractable, 

 but it does not affect the main position.] 



THE AMERICAN ASSOCIATION 



"\A/E are indebted to the courtesy of the Kditor of Science for 

 the following reports of the Sectional proceedings of the 

 American Association. 



In the Section of Physics a paper was read on " The Rela- 

 tion of the Yard to the Metre," by Prof. William A. Rogers, who 

 has given his life to perfecting the construction and the testing 

 of standards of length, and the result of this his latest investi- 

 gation is that the metre is 39/37027 inches in length. One of 

 the most important physical measurements is that of the wave- 

 length of light of any given degree of refrangibility, and this 

 determination is best made by means of the diffraction grating. 

 On account of the extensive use of the magnificent gratings 

 constructed by Prof. Rowland for this purpose, Prof. Rogers 

 instituted an investigation to determine the coefficient of ex- 

 pansion of the speculum-metal used in the construction of these 

 gratings. He also noted that from its homogeneity, fineness of 

 grain, and non-liability to tarnish, this speculum-metal is pecu- 

 liarly suitable for constructing fine scales, though its extreme 

 brittleness is an objection to its use for large scales. 



Prof. Rowland stated that he proposed to construct scales on his 

 ruling-engine which would enable the physicist at any time, by 

 purely optical means, and without knowing the coefficient of 

 expansion of the metal or its temperature, to obtain the value 

 of the length of the scale in terms of the wave-length of any 

 given ray of light. These scales were simply to be straight 

 pieces of speculum-metal ruled with lines just as an ordinary 

 grating, except that the length of the lines is to be only about 

 one centimetre, every one-hundreth line being somewhat longer 

 than its neighbours : the whole ruled slip is to be one decimetre 

 in length. From the manner of ruling, it will be easy to count 

 the whole number of lines in the length of the strip, and then 

 by a simple use of the scale as a grating, in a suitable spectro- 

 meter, the whole length may be immediately found at any time 

 in terms of any specified wave-length of light. In some forms 

 of telephones and in the microphone the action depends on the 

 change in resistance of a small carbon button on being sub- 

 jected to pressure. There has been much discussion as to 

 whether this diminution of the resistance with pressure is due to 

 a change in the resistance of the carbon itself, or simply to the 

 better contact made between the carbon and the metallic con- 

 ductor when the pressure is applied. 



Prof. Mendenhall has carried out some experiments to deter- 

 mine the question ; and one of his methods of experimenting — 

 that with the hard carbons — appears to point conclusively in 

 favour of the theory that the resistance of the carbon itself is 

 altered by pressure. The experiments made by him on soft 

 carbon are open to criticism, though they also point to the 

 change taking place in the carbon. Prof. Mendenhall finds that 

 the resistance is not simply proportional to the pressure, and 

 thinks that by increasing the pressure a point of maximum con- 

 ductivity would be reached where there would be no change in 

 resistance for a small change in pressure. 



Prof. A. Graham Bell, the inventor of the telephone, read a 

 paper giving a possible method of communication between ships 

 at sea. The simple experiment that illustrates the method which 

 he proposed is as follows : — Take a basin of water, introduce 

 into it, at two widely-separated points, the two terminals of a 

 battery circuit which contains an interruptor, making and break- 

 ing the circuit very rapidly. Now at two other points touch the 

 water with the terminals of a circuit containing a telephone. A 

 sound will be heard, except when the two telephone terminals 

 touch the water at points where the potential is the same. In 

 this way the equipotential lines can easily be picked out. Now, 



to apply this to the case of a ship at sea : Suppose one ship to 

 be provided with a dynamo-machine generating a powerful 

 current, and let one terminal enter the water at the prow of 

 the ship, and the .other be carefully insulated, except at 

 its end, and be trailed behind the ship, making connec- 

 tion with the sea at a considerable distance from the vessel ; 

 and suppose the current be rapidly made and broken by 

 an interruptor ; then the observer on a second vessel pro- 

 vided 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 Prof. Bell has actually tried on 

 the Potomac River with two small boats, and found that at a 

 mile and a quarter, the farthest distance experimented upon, the 

 sound due to the action of the interruptor in one boat v\ as dis- 

 tinctly audible in the other. The experiment did not succeed 

 quite so well in salt water. 



Prof. Trowbridge then mentioned a method which he had 

 : some years ago for telegraphing across the ocean with- 

 out 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 connection with the 

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

 sensitive galvanometer. Then any change in the current in the 

 American line would produce a corresponding change in current 

 in the European line ; and thus signals could be transmitted. 



Mr. W. H. Preece 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 sus- 

 pension in the action of the regular cable communication. 

 The instruments used were a telephone in one circuit, and 

 in the other about twenty-five Leclanche cells and an inter- 

 ruptor. 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 Ryde ; 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 be- 

 tween the terminals at Portsmouth and Ryde 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, choosing 

 the 1 .uints 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 con- 

 ductor 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, tin- 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. Prof. Hall, who is now at 

 Harvard, has made some careful experiments to test this expla- 

 nation 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, bring- 



