PEIRCE. 



MANNER OF GROWTH OF A CURRENT. 



507 



ballistic galvanometer, out of a pocket at the middle of the gap. The 

 cylindrical arms of the magnet were held firmly by massive yokes outside 

 the frame, but it was necessary to insert strips of non-magnetic material 

 in the gap to prevent it from gradually closing by the bending of the 

 frame when strong currents were used. These "chocks" were usually 

 bo inserted, by aid of gauges made for the purpose, that just one half of 

 the gap — divided by a vertical line from the other half — was free. 

 Counting from the top of the diagram, the full curves correspond to 

 gap-widths of 1.6 mm., 



6.6 mm., 9.8 mm., and 

 19.7 mm. respectively; 

 between the first two 

 others are bits of two 

 curves belonging to gap- 

 widths of 3.2 mm. and 



4.7 mm. 

 The length of the line 



of induction which goes 

 through the centre of 

 the pole pieces is about 

 250 cm., and it would be 

 easy to find the form of 

 a curve, similar to those 

 of Figure 2, for a closed 

 gap by shearing 3 the 

 upper curve of the dia- 

 gram in the usual man- 

 ner. When the gap was 

 closed, a number of 

 turns of insulated wire 

 were wound directly 

 about the core, and the 



ends of this coil were connected through an oscillograph which made 

 its records on the same piece of paper which recorded the indications 

 of another oscillograph in the main circuit. The main circuit contained 

 also a massive rheostat of 200 ohms, total resistance, and the current 



8 J. Hopkinson, Tlie Philosophical Transactions, 1885 ; J. and E. Hopkinson, 

 The Philosophical Transactions, 1886 ; E. Hopkinson, Report of the Brit. Assoc. 

 Adv. Sci. 1887 ; Ewing, Magnetic Induction in Iron and other Metals, ch. x ; 

 Du Boi9, Philosophical Magazine, 1890. 



Figure 2. 



