324 COMPARISON OF RESISTANCES. 



considered as regular or proportional to the length. Finally, the point 

 of contact with the brass cylinder is very badly defined ; the uncer- 

 tainty is much greater than with the usual methods of measuring. 



Jacobi's rheostate* has in some respects great advantages over that 

 of Wheatstone. There is only one insulating cylinder, on which the 

 wire is permanently coiled, and which rotates about its axis ; a roller 

 which moves parallel to the axis is pressed against the wire by a 

 spring and makes contact. The motion of the roller is defined by 

 that of the cylinder, and moves through one turn of the wire when the 

 cylinder makes a turn. In this way any deformation of the wire is 

 avoided, and the point of contact with the wire is more strictly defined, 

 but the resistance at the point of contact is still very variable. This is 

 the principal drawback to the use of rheostates as measuring instru- 

 ments ; they are however very convenient if we wish to vary the 

 resistance continuously, without wanting to know the exact value. 



These cylindrical rheostates may be replaced by the wire rheostates 

 of Pouilletf and of Poggendorff. J Two platinum wires are stretched 

 parallel to each other; a sliding contact connects the two wires at 

 any point, and allows the introduction of a given length into the 

 circuit. The double contact is generally obtained by means of a 

 solid piece of metal in which are hollowed out two mercury cups ; 

 each of the wires passes through a fine hole drilled in the mercury 

 cups, capillarity being sufficient to prevent the mercury from flowing 

 out. Nevertheless with accurate measurements it is difficult to 

 ensure that the connection of the wire with the mercury always takes 

 place at exactly the same point of the sliding contact. 



Part of these inconveniences may be avoided by stretching the 

 two wires in an upright glass tube, which is more or less filled 

 with mercury from a vertical reservoir. By means of suitable 

 binding screws the two wires can be brought into the circuit either 

 separately or in multiple arc, so as to obtain very unequal degrees 

 of sensitiveness. We may thus get the temperature of the wire by 

 immersing them in a badly conducting liquid, such as petroleum. 



931. COMPARISON OF RESISTANCES BY THE RATIO OF CURRENTS 

 OR OF ELECTROMOTIVE FORCES. The simplest method, at any rate 

 in theory, for comparing two resistances, is to compare the currents 

 which the same electromotive force E gives in the circuits of which 

 they successively form part. 



* JACOBI. Pogg. Ann., Vol. LIV., p. 340. 1841. Vol. LIX., p. 145. 1843. 

 t POUILLET. Elements de Phys. Experim., Third edition, Vol. I., p. 585. 1837. 

 + POGGENDORFF. Pogg. Ann., Vol. LII., p. 511. 1841. 



