108 
Proceedings of the Royal Society of Edinburgh. [Sess. 
the rod to adjust the level. A horizontal brass base-plate B carried centres 
on which the magnets were pivoted, namely, brass uprights with sewing- 
needle points at the top. The centres could be set at any desired distance 
apart. The base-plate was mounted on a horizontal brass turntable between 
the Helmholtz coils, which stood vertically (fig. 7, PI. I). This allowed 
the line of centres to be turned into any azimuth, and the turntable was 
provided with a graduated edge for reading the angle between the line of 
centres and the direction of the magnetic field. The coils stood so that 
their field coincided in direction with the horizontal component of the 
earth’s magnetic field ; the earth’s field could therefore be allowed for by 
making a constant addition to the actual current. 
In tests for rupture two methods were employed. In one, the azimuth 
of the line of centres was set at a definite value of a, and the current was 
slowly increased by means of a potentiometer slide until rupture occurred. 
the base-plate being gently tapped to eliminate effects of friction on the 
pivots. In the other method, the current was kept constant, the turntable 
was slowly turned into a more favourable direction for rupture, and the 
angle a was noted at which rupture occurred. The two methods gave 
extremely concordant results. For values of a approaching 180° the second 
method was in general the more convenient. 
14. By way of illustration the following particulars are quoted for one 
pair of magnets in which the moment was 104-2 C.G.S. units, and the 
length, from centre to centre of the balls, was 6*78 cms. The distance 
between the pivots was, in this test, 7-93 cms., making a^S’965. The 
magnetic half-length r was found under these conditions to be 3*36 cms. 
(which was 0’03 cms. shorter than the half-length measured to the centre 
of the balls), and accordingly i was 1-18. The smallest value of a at which 
any instability was observed was 22°"5. The currents which just sufficed 
to produce instability were measured for a large number of values of a, and 
(after adding an appropriate constant for the earth’s field) are shown in 
relation to a on the curve marked 7 = 1*18 in fig. 8. In the same figure 
corresponding curves are given for the same pair of magnets with two other 
