JiUy 2 2, 1875] 



NATURE 



237 



resistance, R, by the amount of decomposition effected in a 

 voltameter in a given time, and by means of a battery of known 

 elect romotive force. 



Practically, however, such a result would be of no value, 

 because the electromotive force of the battery is counteracted by 

 the polarisation, or electrical tension, set up between the elec- 

 trodes of the voltameter, which depends upon the temperature 

 an d concentration of the acid employed, and upon the condition 

 of the platinum surfaces composing the electrodes. The resist- 

 ance to be measured would, moreover, comprise that of the 

 voltameter, which would have to be frequently ascertained by 

 other methods, and the notation of time would involve consider- 

 able inconvenience and error. For these reasons the voltameter 

 has been hitherto discarded as a measuring instrument, but the 

 disturbing causes just enumerated may be eliminated by com- 

 bining two similar voltameters in one instrument, which I pro- 

 pose calling a " differential voltameter," and which is represented 

 in the accompanying drawing. 



It consists of two similar narrow glass tubes, A and B, of about 

 2 '5 millimetres in diameter, fixed vertically to a wooden frame, 

 F, with a scale behind them divided into millimetres or other 

 divisions. The lower ends of these tubes are enlarged to about 

 6 millimetres in diameter, and each of them is fitted with a 

 wooden stopper saturated with paraffin and pierced by two 



Fig. 5. 



platinum wires, the tapered ends of which reach about 25 milli- 

 metres above the level^ of the stopper. These form voltametric 

 electrodes. 



From the enlarged portion of each of the two voltameter tubes 

 a branch tube emanates, connected, by means of an india-rubber 

 tube, the one to the moveable glass reservoir G, and the other to 

 g'. Fig. 5. These reservoirs are supported in sliding frames by 

 means of friction springs, and may be raised and lowered at 

 pleasure. The upper extremities of the voltameter tubes are cut 

 smooth and left open, but weighted levers, L and l/, are provided, 

 with india-rubber padr, which usually press down upon the open 

 ends, closing them, but admitting of their beir.g raised, with a 

 view of allowing the interior c f ihe tubes to be in open communi- 

 cation with the atmosphere. Ilavirg filled the adjustible reser- 

 voirs with dilute sulphuric acid on opening the ends of the volia- 

 meter tubes, the liquid in each tube will rise to a level with that 

 of its respective reservoir, and the latter is moved to its highest 

 position before al '.-wing the ends of the tubes to be closed by 

 the weighted and padded levers. 



The ends of the platinum wire forming the electrodes may be 

 platinised with advantage, in order to increase the active surface 

 for the generation of the gases. 



Fig. 6 represents the connections of the voltameter with the 

 pyrometer. One electrode of each voltameter is connected with 

 a common binding screw, which latter may be united, at will, to 

 either i)ole of the battery, vhibt the r< ma'uing two e'ec'rodes 

 are, at the same moment, connected with the other pole < f tlie 



same battery ; the one through the constant resistance- coil, x, 

 and the other through the unknown resistance, x'. This un- 

 known resistance, x', is represented to be a pyrf meter-coil. 



By turning the commutator seen at Fig. 5 e'tVer in a right or 

 left-hand direction from its central or neutral position (in which 

 position the contact-springs en either side rest on ebcni'e), the 

 current from the battery flows through the two circuits, causing 

 decomposition in the voltameters ; and the gases generated upon 

 the electrodes accumulate in the upper portions of the graduated 

 tubes. By turning the commutator half round every few seconds, 

 the current from the battery is rever ed, which prevents polarisa- 

 tion of the electrodes, as already stated. 



The relative volumes, v and r', of the gases accumulated in an 

 arbitrary space of time within each tube must be inversely pro- 

 portional to the resistances, R and A", of the branch circuits, 

 because — 



R A" ' 

 and, therefore, 



z' : t/' = A" : A'. 

 The resistance^ R and R' are composed, the one of the resistance 



C, plus the resistance of the voltameter A, and the other of the 

 unknown resistance X, plus the resistance of the voltameter />. 

 But the instrument has been so adjusted that the resistances of 

 the two voltameters are alike, being made as small as possible, 

 or equal to about I mercury unit, to which has to be added the 

 resistances of the leading wires, which are also made equal to 

 each other, and to about half a unit ; these resistances may 

 therefore both of them be expressed by y. 

 We have, then — 



z.' :v = C + y:X+y, 

 or — 



A'=l(C+7) 



(0 



which is a convenient formula for calculating the unknown 

 resistance from the known quantities Card 7, and the observed 

 proportion of v and v'. 



The constant of the instrument ('^) is easily deternuned, fiom 

 time to time, by substituting a known resistance for A", and 

 observing the volumes, v and </, after the current has been actng 

 during an arbitrary space of time, when in the above f< rmula, y, 

 has to be separated as the unknown quantity, giving it the 

 form — 



y-±^Ei:'^ (^) 



Tl e ccnditicn of er.urli'v 1 etwetn tl e int(inr.l nsislr.KCS of 



