Page:Outlines of Physical Chemistry - 1899.djvu/210

 192 OUTLINES OF PHYSICAL CHEMISTRY

electromotive force of which must be greater than that of the element to be measured. It is closed by a circuit con- taining a resistance-box r and a fixed resistance a b. The potential-difference at the points a and b depends on the electromotive force of the element e and of the auxiliary resistance b.

The secondary circuit, a 7r /3, contains the element t and a galvanometer or an electrometer. One of the extremities a is fixed at a, whilst the other can slide along the graduated resistance a b by a movable contact ft.

If the movable contact is at the point a, then no current will be sent through the branch aw ft by e. But as j3 is moved to the right the difference of potential between the

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points a and ft becomes greater, and the current which passes through a tt/3 increases in intensity. If the element 7T be now put in opposition to this current and if /3 be moved along a b, when it reaches a certain point the galvanometer will show that no current is passing through the branch a tt /3, so that the opposite potentials are exactly equal. If equilibrium of the potentials takes place, when the element •* is in the circuit, when the movable contact is at the point ft of the resistance a b, and if the same is the case when the contact is at ft with a standard element *r k place of tt, it is evident that the electromotive forces of the two elements are as the lengths a ft and a ft. The equation a ft : a /3 = e : e contains only one unknown term

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