The balance is adjusted by shunting either AD or BD with a box, S, containing 20 to Ioo ohms. All the wires in the quadrilateral must be of the same metal as AB, to avoid accidental thermoelectric effects which would obscure the result.

Io) so that BD was equal to r, he produced it E only so far that, when the extremity D' was joined with circle was equal to r; in C, the part D'F outside the A o FIG.

Rod as in Foucault's construction, and BD is another rod which can be set to the direction in which the rays are to be reflected.

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Then MA'B'N is a right trapezium, whose area is equal to that of Cabd; and it is related to the latter in such a way that, if any two lines parallel to AC and BD meet AB, CD, MN, A'B', in E, G, P, E', and F, H, Q, F', respectively, the area of the piece PE'F'Q of the right trapezium 'B.

In the case of oblique reflection at an angle cb, the error of retardation due to an elevation BD (fig.