114 Mr. 0. Heaviside on the best Arrangement of 



In the place of the forked tube into which in all the fore- 

 going experiments the note or sound was introduced, we can put 

 two separate tubes of exactly equal length and form, each con- 

 sisting of three separate pieces inserted one in another and 

 capable of being turned round so that we can move the two 

 openings at their ends in any direction we please without altera- 

 tion of the length of the tube or of the form of its turnings. 

 This arrangement permits then the entrance of the note of two 

 different points of a vibrating body into the apparatus — for 

 example, of two vibrating bridges of a plate with contrary signs, 

 or of the same place on its opposite surfaces : in both these cases 

 the interference takes place when the two paths are equal, and 

 the tone first becomes audible when the interference is destroyed 

 by lengthening one of the compound tubes. 



In order to adapt the apparatus to the demonstration of the 

 wave-lengths of a note in different gases, and for the experiments 

 of Zoch, I have provided the pipes with two cocks, which serve to 

 fill and empty them. Of course, if we experiment with any other 

 gas than atmospheric air, the resonator cannot remain in direct 

 communication with the interior of the pipe ; and therefore we 

 must in that case place between them a small cavity, which is 

 divided in the centre by a thin membrane into two halves — the 

 one to be united with the pipe, the other with the resonator. 

 Besides we must then have india-rubber rings to draw over the 

 ends of the tubes which are only placed within each other, so 

 that the gas cannot escape at these places. 



It is, of course, understood that this apparatus permits the 

 direct observation of different interference-phenomena by the ear, 

 and consequently the repetition of the experiments of Mach, 

 Quincke, and others. For this purpose we have but to place 

 one of the forked tubes before the apparatus and connect the 

 former with the ear by an india-rubber tube. 



XV. On the best Arrangement of Wheatstone's Bridge for mea- 

 suring a given resistance with a given Galvanometer and Battery. 

 By Oliver Heaviside, Great Northern Telegraph Company, 

 Newcastle-on- Tyne*. 



IN the figure, a, b, c, and d are the four sides of the electrical 

 arrangement known as Wheatstone's bridge or balance, e 

 the galvanometer, and/ the battery branch. Throughout this 

 paper d is supposed to be the resistance to be measured, and e 

 and / both known. The problem is to find what resistances 

 should be given to the sides a, b, and c (which we are able to 



* Communicated by th& Author. 



