LESSONS IN ELECTRICITY. 



327 



conductor depended upon its surface, 

 and not upon its solid content*. An 

 anvil weighing 200 pounds gave a smaller 

 sp::rk tlan a speaking trumpet weighing 

 10 pounds. A solid ball of lead gave a 

 spark only of the same force as that ob- 

 tained from a piece of thin lead of tho 

 same superficies, bent into the form of a 

 hoop. Finally Monnier obtained a strong 

 spark from a long strip of sheet lead, 

 but a very small one when it was rolled 

 into a, lump. 



Le Roi and D'Arcy showed that a hol- 

 low sphere accepted the same charge 

 when empty as when filled with mercury, 

 which augmented its weight sixty-fold. 

 And this proves the influence of surface 

 ad distinguished from mass. 



The distribution of electricity is well 

 illustrated by the deportment of hollow 

 bodies. Impart by your carrier (fig. 15) 

 successive measures of electricity to the 

 interior of an insulated ice-pail, or a 

 pewter pot. On testing the interior of 

 the vessel with the carrier and an elec- 

 troscope no electricity is found there ; 

 but it is found on the external surface. 

 A hat suspended by silk strings answers as 

 well as the ice-pail. 



Thi* ^Tperiment with the hat is a very 

 inatractire one. Tho hat may be charged 

 cither with Cottrell's rubber or with your 

 rubbed glass tube. 



Notice, Trhen testing, that you take 

 your strongest charges from the cd^ss 

 and not from tho round or flat .smface cf 

 the hat. The strongest charge of all U 

 communicated to the earlier by the IG? 

 of the hat. 



The successive charges may be cow- 

 muni cated to the hat by a metal ball :u-:> 

 pcndcd by silk. The charged bll, o* 

 touching the interior surface, beeorr>4 

 completely unclectric. 



Franklin placed a long chain in -A sti- 

 ver tea-pot which he electrified. Con- 

 necting his teapot with a pith-ball ch -.- 

 troscope he produced a diverge*.:*. 

 Then lifting the chain by a silk string**. 

 found that over the portion outside v~$ 

 teapot the electricity diffused itself, T!M 

 withdrawal of the electricity from !*t* 

 electroscope being announced by the par- 

 tial collapse of the divergent pith-balls. 



The mode of repeating lhi experi y iiicjj 

 is shown in fig. 53, where T is the tea- 

 pot, supported on a ^ood glass tumbler 

 o, and connected by the wire w with the 



