300 



LESSOMS IN FLTCTRTCITY. 



Regarding the quality of the electricity 

 of the flannel or of the silk rubber, the 

 attraction of the lath teaches you nothing. 

 But, suspend your rubbed glass tube, and 

 bring the flannel rubber near it : repul- 

 sion follows. The silk rubber, on the 

 contrary, attracts the glass tube. Sus- 

 pend your rubbed gutta-percha tube, and 

 bring the silk rubber near it ; repulsion 

 follows. The flannel, on the contrary, 

 attracts the tube. 



The conclusion is obvious : the elec- 

 tricity of the flannel is positive, that of 

 the silk is negative. 



But the flannel is the rubber of the 

 gutta-percha, whose electricity is nega- 

 tive ; and the silk is the rubber of the 

 glass, whose electricity is positive. Con- 

 sequently, wo have not only proved the 

 rubber to be electrified by the friction, 

 but also proved the electricity of the 

 rubber to be opposite in quality to that 

 of the body rubbed. 



All your subsequent experience will 

 verify the statement that the two electric- 

 ities always go together ; that you can- 

 not excite one of them without at the 

 wine time exciting the other, and that 

 the lectricity of the rubber, though op- 

 posite in quality, is in all cases precisely 

 equal in quantity to that of the body 

 rubbed. 



And now we will test these principles 

 by a new experiment. In 5 we learned 

 that an ebonite comb is electrified by its 

 passage through dry hair. You can 

 readily prove the electricity of the comb 

 to be negative. But the hair is here the 

 rubber, and, in accordance with the 

 principle just laid down, an equal quan- 

 tity of positive electricity has been excit- 

 ed in the hair. If you stand on the 

 Ground uninsulated, the electricity of the 

 hair passes freely through your body to 

 the earth. 



But stand upon an insulating stool* 

 on your board, for example, supported 

 by four warm tumblers while I, stand- 

 ing on the ground, pass the comb briskly 

 through your hair. I pass it ten, twenty, 

 tliiity times, and then ask you to attract 



* A stool with glass legs wnieh, to protect 

 them f:om the moisture of the air. are 

 usually coated with a solution of shellac. 

 Regarding the attraction of glass for atmos- 

 pheric humidity, you will call to mind what 

 Las been said in 5. 



your balanced lath. You present your 

 knuckle, but there is no attraction. 



Here the comb and the hair soon reach 

 their maximum excitement, bevond which 

 no further development of electricity oc- 

 curs. Now, though the comb, as shown 

 in 5, is competent to attract the lath, 

 while your body is here incompetent to 

 do so, this may be because the small 

 quantity of electricity existing in a con- 

 centrated form upon the comb becomes, 

 when dirfused over the body, too feeble 

 to produce attraction. 



Can we not exalt the electricity of your 

 body ? Guided by the principles laid 

 down, let us try to do so. First I pass 

 the unelectrified comb through your 

 hair ; it comes away electrified. After 

 discharging the comb by passing my hand 

 closely over it, I pass it again through 

 the hair. As before, it quits the hair 

 electrified, and I again discharge it. I 

 do this ten or twenty times, always de- 

 priving the comb of its electricity after it 

 has quitted the hair. Now present your 

 knuckle to the balanced lath. It is pow- 

 erfully attracted. 



Here, as I have said, the unelectrified 

 comb carried in each case electricity away 

 with it ; but, in accordance with the fore- 

 going principles, it left an equal quantity 

 of the opposite electricity behind it. 

 And though the amount of electiicity 

 corresponding to a single charge of the 

 comb, when diffused over the body, 

 proved insensible to our tests, that 

 amount ten or twenty times multiplied 

 became not only sensible, but strong. 

 Indeed, by discharging the comb, and 

 passing it in each case undeetrified 

 through the hair, the insulated human 

 body cair be rendered highly electrical. 



Near the beginning of this section I 

 said, in rather an off-hand way, thai 

 rubbed flannel repels rubbed glass, while 

 rubbed silk repels rubbed gutta-percha. 

 Now, while it is generally easy to obtain 

 the repulsion by the flannel, It is by no 

 means always easy to obtain the repulsion 

 by the silk. Over and over again I have 

 been foiled in my attempts to show this 

 repulsion. I wish you, therefore, to be 

 awaro cf an infallible method of obtain- 

 ing it. 



Stand on your insulating stool, and rub 

 your glass tube briskly with the amalga- 

 mated silk ; hand me the tafcc. I pass 



