Galvanometers of High Sensibility. 253 



OP and less than OA, and one of the periods increases and 

 the other diminishes as OC approaches AO in magnitude. 



Very light systems are of course extremely sensitive to air 

 currents, such as will be produced by very slight inequalities 

 in temperature ; it is therefore important that the entire gal- 

 vanometer be protected somewhat as indicated in fig. 2. The 

 coil faces are covered with gold-leaf — and static charges of elec- 

 tricity upon them have given little or no trouble ; but magnetic 

 impurity in the material of the coils is very troublesome, and 

 necessitates a free space of at least a millimeter between the coil 

 faces (see Nichols, " The Galvanometer"). White silk insulation 

 is somewhat better than green, though still very bad ; and a part 



Fig. 4 



. s. 



\ AO= Earth's Mag. Force. 



\ OC, OD = Control Mag. 

 *. Force. 



| AC, AD = Resultant Mag. 

 / Force, 



/ 

 / 



/ 

 / 



of the difficulty is with the copper itself. The trouble seems to 

 be in part due to a permanent magnetization of the coils, pro- 

 ducing sometimes a rather complicated distribution of poles 

 over the coil faces, which can be largely removed by a careful 

 demagnetization of the coils ; and in part to the poles induced 

 by the approach of the magnets of the needle system itself — 

 which can only be helped by using purer material. 



In winding the coils the ends of all the sections were 

 brought out, and small soldered junctions made on the back of 

 the coil ; hence six sizes of wire, viz. — 40, 38, 36, 34, 32, 30, 

 could be used without danger of wasting space in junctions. 

 But the gain is probably hardly worth the trouble. 



In making very light systems it is convenient to begin by 

 fastening the magnets in the proper relative position, to a piece 

 of plate glass with a solution of sugar or flour paste ; after the 

 glass staff and mirror have been fastened on, the system can 

 be loosened with a drop of water. One of our best systems 

 developed after use a second position of equilibrium, which it 

 would assume if given a large deflection ; it seemed to be due 

 most probably to a change in strength of the magnets caused 

 by the inductive action of the field, rather than to torsion of 

 the staff of the system — as before suggested.* 



*Astrophys. Jourual, Jan., 1901, p. 47. 



