58 M. E. Stohrer on an Improved Induction Apparatus. 

 with auother. The central cylinder of the coils consists of pieces 

 Fis-. 1. 



of paper glued over each other and saturated with varnish. The 

 wails of the coil are of wood, the best for this purpose being sound 

 pear-tree wood ; they are only 3 millims. thick, and have a dia- 

 meter of 100 millims. In winding the fine wire and its insulation, 

 the greatest care is known to be necessary. Although, by making 

 three compartments, an approximation of the parts of the induc- 

 tion wire which lie at a great distance from each other is avoided, 

 still the good spinning and good varnishing of the wire is by no 

 means sufficient. It is easy to see that small sparks pass from 

 one layer to that above it, when the excited spiral is observed in the 

 dark. There is no means of avoiding this, except by adding to 

 the varnished covering which surrounds the wires another coating 

 which shall be impervious to such sparks. Fluid insulators in- 

 deed, such as oil of turpentine, close up the ruptured place im- ' 

 mediately ; but little is gained by this, as the passage of the 

 sparks will not thereby be prevented. If one layer of wire be 

 separated from the other by gutta-percha, wax-paper, &c., the 

 sparks make a path for themselves round the edges and along 

 the sides of the coil. Professor Poggeudorff made use of a light 

 fluid wax, which has been of greater service. The coils of the 

 apparatus now in question are wound and insulated in the fol- 

 lowing manner : — A. small glass tube is introduced at the inner 



