Page:Outlines of Physical Chemistry - 1899.djvu/212

 194 "' OUTLINES OF PHYSICAL CHEMISTEY

since they are directly connected with a thick wire. But as we go towards the right or towards the left the poten- tial-difference increases. If we consider, for example, the volt.

Suppose now that in the branch circuit (fig. 44) we have:

1. The element w under examination, in opposition to the principal current coming from e.

2. A Lippmann electrometer, l. 8. If necessary, one or more standard 1-volt elements,

a, b, c, d, e, /, in opposition to the element w.

4. A commutator by which the electrometer can be closed on itself or by which the circuit can be closed by interposing the electrometer.

The ends a and /3 of the secondary circuit are so con- nected on the resistance a b that when the circuit is closed of equilibrium. One succeeds in this after a few trials. respectively as in the figure, then the electromotive force measured is 1*16 volt, because it is necessary to take the standard 1-volt element a, which is in opposition to *-, into consideration.

Measurement of Besistance :

Wheatstone Bridge Method

The arrangement of the Wheatstone bridge is diagram- matically represented in fig. 45. At the points a and b the potentials are p and p', and two points c and d having the same potential p are connected by the bridge (in which there is a galvanometer) ; and let us suppose that the resistances of the four branches thus formed are r and /, b and b'.

��standard cell is so arranged that its current is opposed to the current from e. The resistance b is then altered until the electrometer denotes potential-equilibrium at a and b.

�� �