224 LECTURES TO SCIENCE TEACHERS. 



with spirits of wine. Now if I move this glass rod slightly 

 electrified, or this electrified stick of sealing-wax, in the 

 vicinity of the aluminium arm, you see at once the effect 

 of external electrification on the movable arm, which 

 corresponds exactly to the movable arm in the torsion 

 balance. I will now bring the glass rod near to the 

 outside of the bell-jar, and draw this slip of paper over 

 the surface of the bell- jar in the vicinity of the glass 

 rod, and then carry away the glass rod. You see that I 

 have left the bell-jar itself with a portion of its outer sur- 

 face electrified in such a way that it attracts the needle 

 round through a right angle. 



I will now wet the outside of the bell-jar with a sponge 

 and thus make the outside of the glass a fairly good 

 conductor. By this the needle is partly, but as you see not 

 entirely, screened from the influence of external electri- 

 fication. Yet the natural dampness of the glass cases, 

 the dust and so forth that may be upon them, is the 

 whole protection that the electroscopes and electrometers 

 ordinarily constructed have against such disturbing 

 influence. 



Faraday pointed out that nothing but a complete metallic 

 protection is sufficient. He protected the whole of the 

 interior of the electroscopes and electrometers that he used 

 with slips of tinfoil pasted on the inside surface of the 

 glass cases, leaving only such spaces as were necessary for 

 looking through. In the instruments of Sir William 

 Thomson that I shall have to show you the movable 

 parts are protected by brass covers, tinfoil, or by a wire 

 cage which is often found to be sufficient and to be very 

 convenient. 



Another improvement that Faraday made with respect to 

 the torsion balance was the introduction of what is called the 

 heterostatic method of using the instrument. Electrometers 

 may be distinguished into two classes, idiostatic and hetero- 

 static. In the idiostatic class the whole electric force 

 depends on the electrification which is itself the subject of 

 the test. In the heterostatic class, besides the electrification 

 to be tested, another electrification, maintained indepen- 

 dently of it, is taken advantage of. The torsion balance as 

 used by Coulomb was an idiostatic instrument. The elec- 

 tricity to be tested was divided each time between the balls 



