This is calibrating your set by the known wave-lengths of distant stations.
With this object a scale is ruled on the machine, and the errors of the uncorrected screw are determined by calibrating the scale.
This compensating arrangement is also helpful, as has been said before, for calibrating the E.M. value of the deflection.
The wood is placed in the water, and known weights are used on the cardboard while calibrating.
For calibrating the thermometers, an ordinary wooden water-pail is provided with several holes in the side near the bottom.
For calibrating these thermometers a special arrangement is necessary.
Equally important with the foregoing is the necessity of calibrating and testing of all thermometers used during a test.