36 Dr. H. Pender on the Magnetic 



found more advantageous to sacrifice sensibility to steadiness 

 of the needle, so that the galvanometer was usually adjusted 

 to have only about one and a half times the sensibility of 

 last year, a current of *7 x 10~ 10 amperes giving a deflexion 

 of 1 mm. on a scale 2 metres distant. At this sensibility the 

 spot of light was ideally steady, the zero position seldom 

 varying more than 2 mm. during the time required for the 

 determination of a deflexion (the period of the needle was 

 about 35 seconds), although there was sometimes a slow drift 

 of the light to the left. Any error due to this effect was 

 easily eliminated by taking a reading first on one side of the 

 zero position and then on the other. 



On the shaft of the combined reverser and commutator 

 which served to reverse the sign of the charge on the disks 

 and to commutate the galvanometer terminals, a second re- 

 verser was mounted and connected in series with a DanielPs 

 cell, resistance-boxes, and the test-coil T on the disk appa- 

 ratus. My idea was to adjust the value of the current 

 through this coil until its effect on the large coil I between 

 the disks was just equal and opposite to the effect of the 

 moving charged disks, thus employing a zero method. It 

 was found more convenient, however, to note first the de- 

 flexion produced by passing a known current through the 

 test-coil with the disks discharged, then to break the circuit 

 of the conduction current, connect in the Voss machine so as 

 to charge the disks, and note the deflexion produced by the 

 convection current, then again note the effect of the con- 

 duction current, and so on. This was accomplished by means 

 of a set of switches operated by the observer at the galvano- 

 meter. The conduction current was adjusted so as to give 

 about the same deflexion as the convection current. Its 

 value was determined from the known resistance of the 

 circuit and the E.M.F. of the Daniell cell. The zero method 

 was abandoned on account of the great length of period of 

 the galvanometer needle, nearly a minute being required to 

 detect any slight variation from the zero position. 



The above method of procedure obviated the necessity of 

 determining the sensibility of the galvanometer and the 

 speed of the reverser for each set of readings, and also the 

 constant u A " of the former paper. Much trouble was ex- 

 perienced in getting brushes for the conduction current 

 reverser which would make a steady contact, but finally this 

 difficulty was overcome by making the brushes of very soft 

 copper foil, each brush consisting of ten layers of foil. It 

 was necessary, in the course of the experiments, to replace 

 these brushes several times. 



