of High Specific Resistances. 457 



considerable no distortion of them ought to be caused by the 

 gum, if the ordinary precaution of allowing them to cool 

 slowly be observed. 



In the experiments hitherto made the cooling took place 

 during the night in the gas-oven, which being coated with 

 non-conducting material took a long time to cool. On the 

 other hand, in all experiments of the sort one is in a dilemma. 

 If the substance is placed between conducting-plates there are 

 dangers of the kind mentioned; if, on the other hand, the 

 material itself be worked with a view to making it take a 

 prescribed form, the difficulties, especially in the measurement 

 of its thickness, become great. I began by making some 

 attempts of this kind, using blacklead. to make the gum-sur- 

 face conducting, and plating this electrolytically. The diffi- 

 culties arising in the shaping of the plates are, however, 

 practically prohibitive with friable material. Besides this I 

 have often noticed that in electrotyping it is difficult to 

 prevent the deposit being "spotty" at first, and this has 

 shaken my faith in the perfect continuity of ordinary black- 

 lead surfaces. Possibly platinating with an induction-coil 

 may be really the best way. 



However, to finish the description of the case in point : — 

 The gum was carefully scraped away from the edges of the 

 plates as soon as they were cool, and the screws were screwed 

 back. This could not be done with any ease at first because 

 of the cementing action of the gum. This was got over by 

 heating the head of the screw w T ith a Bunsen flame : finally, 

 the screws were retracted far enough to be quite out of the 

 way*. A correction to the area of surface has of course to 

 be made for the three screw-holes. If the thickness of the 

 gum be considerable compared with the diameter of the hole, 

 this may be very complicated. In the present case it was 

 negligible. Thus the whole area of the gum-plate was 

 193*04 centim. less the area of the three screw-holes — *7128 

 centim., i. e. 192-3272. 



Now it is clear that, owing to the curvature of the lines of 

 flow round the edges of the holes, the real correction will be 

 less than the one made. The deposit of gum, however, is 

 pierced by a hole corresponding to the conical end. of the screw 

 and, consequently, only very small. 



* Note, Dec. 1888. — The difficulty is, however, serious, and has led to 

 new hollow screws being* made. Through the holes bored down the 

 centres of the screws gold-plated copper rods pass ; these are pinned to the 

 screws till it is required to retract them ; the ends of the screws them- 

 selves are flush with the lower surface of the upper plate, or very near so. 



