'W 



i; 



'»ii 



632 



KEroiiT — ^^1884. 



as to ensure almost absolute accuracy. This method, which is explained, is not- 

 suitable for the second kind of application, where the forces transmitted have to bo 

 considered. The paper discusses the practical mode of dealing with this question, 

 and by means of diagrams and a model (which was exhibited) shows that the trans- 

 mission of moderate forces can be satisfactorily accomplished where accurate 

 velocity ratios are not required. 



4. Oil some Irregularities depending on Temperature in Baihfs experi- 

 ments on the Mean Deiisitg of the Earth, By Professor W. Jil. 



Hicks, 3I.A. 



, , _ — 



5. On Safety Fuses for Electric Circuits. 

 By Trofessor Sir William Tiiomsox, LL.D., F.B.S. 



6. A Lecture E.vperiment on Induction. 

 By Professor Lord Rayleigu, LL.D., F.E.S. 



It is well known that an electro-magnet, Interposed in the circuit of an altev- 

 nato current machine, diminishes the etloct far more than in a degree corresponding 

 to the resistance of the additional wire. This behaviour of an electro-magnet may 

 be exhibited to an audience in an instructive manner, by use of a helix wound with 

 two contiguous wires (sucli as are commonly used for large instruments), one of 

 which is included in the circuit of a I>e Meriteus machine, and a few incandescent 

 lamps. If the circuit of the second wire be open, the introduction of a few stout 

 iron wires into the helix causes a very marked falling olf in the incandescence. On 

 closing the second circuit, currents develope themyelves in it of such a kind as to 

 compensate the self-induction, and the lights recover their brilliancy. l']ven 

 without iron, the effect of closing the second circuit is perceptible, provided the 

 degree of incandescence be suitable. 



An arrangement suitable for illustrating tlie same phenomenon with current^^ 

 of small intensity was described in * Nature ' for May S), 1S?1*. 



7. On Telephoning through a Cable. 

 By Professor Lord Raylkigh, LL.D., F.B.S. 



The principles of tliis subject were laid down thirty years since by Thomson, 

 but the author had not met with an application to the circumstances of tfie- 

 telephone. 



A periodic variation of potential, imposed at one end, is propagated along the 

 line in accordance witii tlie law 



V = e ~>/a« ■ •'■ cos, ("^ — a/.w ••'■)• 

 in which J?-' is the frequency of the electricr.l vibration. For Atlantic caljles 



the constant k, depending upon the resistance and the capacity, has in C.G.S. 

 measure such a value as 2 x lO"'. Tlie distance, in traversing whicli tlie ampU- 

 tude is reduced in the ratio e : 1, is given by 



,2k 2x108 .. , 

 .i = V = — centimetres. 

 n \/n 



If we take a pitch rather more than an octave above that of middle c, we have 

 n ^- 3,600, Vn = UO, so that 



,1' = 3 X 10" centimetres = 20 miles approximately. 



A distance of twenty miles would thus reduce the intensity of sound to almost 

 fi tenth, an operation wliich could not be often repeated without rendering it 



n 



