of Varying Currents in Inductive Circuits. 265 



elevation in fig. 6. The mirror, the system of six magnets, 

 and the aluminium arm are formed into a rigid system by 

 being tied together with thin wire. 



Fig. 6. 



BACK 

 ELEVATION 



END 

 ELEVATION 



It will be noticed that the angular motion of the needle is 

 narrowly restricted. The instrument is, however, intended 

 for use in a "zero" method, and a very small range is all that 

 is necessary. 



When setting up the instrument for use, there are two 

 points which must be particularly attended to : — 



(a) The needle when freely turning must touch the two 

 stops simultaneously, and 



(b) it must only just be kept in contact with its stops when 



no current is flowing in the coils. 



The following is the way in which we perform these adjust- 

 ments : — The instrument, having been approximately levelled 

 and the stops roughly adjusted, a magnet is brought near so 

 as to make the needle press lightly against them. Under 

 these circumstances we find that the movements of the 

 reflected spot of light or, better, of the image in a telescope, 

 afford a most delicate and satisfactory criterion as to whether 

 the adjustment is perfect. An incessant though slight pen- 

 dulous motion of the needle is observed as long as it rests on 

 one stop only : this movement suddenly and entirely dis- 

 appears as soon as the adjustment is perfect; this adjustment 

 is most conveniently effected by means of the front levelling- 

 screw alone. Finally, the magnet which was brought near 

 is gradually withdrawn until the needle just ceases to press 

 against its stops, and the adjustment is then complete. 



Fig. 7 is an example (reduced from a drawing on a much 

 larger scale) of curves obtained by this method. It shows 



both the actually observed points on an I - I curve and also 



the current-curve derived from it. This is the current which 

 occurs on reversing the impressed potential difference at the 

 Phil. Mag. S. 5. Vol. 40. No. 244. Sept. 1895. T 



