274 LECTURES TO SCIENCE TEACHERS. 



elongated conductor such as a cylinder, or still better one 

 which is rounded at one end and pointed at the other. You 

 know that the electrical density at the extremities is greater 

 than at the intermediate parts, and if one end is rounded and 

 the other has a sharp point, the density at the point is very 

 much greater than the density at the round end ; still the 

 electricity does not flow from the point to the round end, or 

 from the ends to the middle. Take again such a case as 

 this. If we charge this jar inside and touch the inside with 

 a carrier-plane, we cannot detect any electrification ; if we 

 touch the outside we find it electrified. If you charge the 

 inside or outside of the jar, and then examine the inside, 

 you get no charge from that, but you do get a charge from 

 the outside, so that examining it in this way the inside 

 of the jar appears to be neutral. Wo connect the jar with 

 the electroscope in a different way by taking an insulated wire, 

 and bringing one end in contact with the outside of the jar ; 

 the leaves of the electroscope diverge, showing that the jar 

 is charged. Bring the end of the wire inside the jar, and the 

 leaves will likewise diverge. Thus, the inside of the jar 

 examined by means of a carrier-plane appears to be neutral, 

 but examined by means of a wire connected with an 

 electroscope, it appears to be charged just as much as 

 the outside. Again, although you cannot discharge the jar 

 by lading out electricity from the inside by a carrier, you 

 can discharge it by touching the outside with a carrier, then 

 uninsulating the carrier, touching the jar again, and repeating 

 the process over and over again. You can discharge the jar 

 in this way from the outside but not from the inside ; but 

 if you use a wire connected with the earth, whether you 

 touch the inside or touch the outside of the jar, you equally 

 discharge it. 



There is another way in which we can show that the 

 inside of the jar although there is no electric density upon it, 

 and it appears neutral, when we examine it by means of a 

 Carrier it is not in the same condition as a space not sur- 

 rounded by electricity. If we put in an insulated ball which 

 is neutral to begin with, the jar being charged on the outside, 

 and, when the ball is inside, touch the ball with a wire in 

 connection with the earth and then take it out, we find the 

 ball charged, although there is no charge on the inside of 

 the jar. Thus, although the space within the jar does not 



