LESSOiVS IX ELECTRICITY. 



59 



This is a fit place to say that you must keep a close eye upon the 

 tumblers you employ for insulation. Some of them, made of common 

 glass, are hardly to be accounted insulators at all. We shall prove this. 



Our mastery over this subject of induction must be complete, for 

 it underlies all our subsequent inquiries. Without reference to it 

 nothing is to be explained ; possessed of it you will enjoy, not only a 

 wonderful power of explanation, but of prediction. We will attack 

 it, therefore, with the determination to exhaust it. 



And here a slight addition must be made to our apparatus. We 

 must be in a condition to take samples of electricity, and to convey 

 them, with the view of testing them, from place to place. For this 

 purpose the little " carrier," shown in Fig. 10, will be found conven- 

 ient. Tib a bit of tin-foil, two or three inches square. A straw stem 

 is stuck on to it by sealing-wax, the lower end of the stem being cov- 

 ered by sealing-wax. To make the insulation sure, the part between 

 R and S' is wholly of sealing-wax. You can have stems of ebonite, 

 which are stronger, for a few pence ; but you can have this one for a 

 fraction of a penny. The end M' is to be held in the hand ; the elec- 

 trified body is to be touched by T, and the electricity conveyed to an 

 electroscope to be tested. 



Fig. 11. 



Touch your rubbed glass rod with T, and then touch your electro- 

 scope : the leaves diverge with positive electricity. Touch your 

 rubbed gutta-percha or sealing-wax with T, and then touch your elec- 

 troscope : the leaves diverge with negative electricity. If the elec- 

 tricity of any body augment the divergence produced by the glass, 

 the electricity of that body is positive. If it augment the divergence 

 produced by the gutta-percha, the electricity is negative. And now 

 we are ready for further work. 



