D2 Mr. 8. C. Laws on the Magnetic Susceptibility 
of the balance and through the shelf on which the balance 
rested. The height of the coil was then adjusted so that the 
Fig. 1. 
middle of the rod—all the rods were 15°6 cms. in length— 
occupied such a position that it rose to the level of the end 
of the coil when the current was put on. The observer then 
seated himself at the telescope and observed the reading of 
the scale occupying the centre of the field of the telescope. 
In practice the suspended mirror was never stationary, 
but by observing in the telescope the scale-readings at the 
ends of a series of successive oscillations, the amplitude of 
which did not exceed 1 cm., the position of the zero was 
readily obtained. 
The current of the required magnitude was then sent 
through the coil, when, owing to the mechanical force 
