Page:Collected Physical Papers.djvu/48

When this is the case the receiver ceases to respond. Let A be the corresponding reading of the platform index. A stationary index I′ is now placed opposite the reading A of the graduated circle.

When the cylinder is rotated in the opposite direction a second reading B for the critical angle is obtained. It is obvious that, neglecting errors, A—B is equal to twice the critical angle.

The platform index is now clamped and the circle as a whole is rotated till B comes opposite to the fixed index I′. The circle is now clamped, the platform arm unclamped, and the central table rotated till another reading C for the critical angle is obtained. Then, as in the previous case, $B-C=2i$, where $i$ is the critical angle. The circle as a whole is now rotated till C comes opposite the fixed index.

Fig. 7. R, the Radiator; C, the Receiver.

Thus at each successive operation the circle is rotated past the fixed index through $2i$. The successive difference of readings of the circle in reference to the fixed outside index, thus gives a series of values of $2i$.

The result will be more accurate if we take the mean readings ${\tfrac {1}{2}}(A+B),{\tfrac {1}{2}}(B+C)$, ...., and take their differ-