ELECTRIC DENSITT. 213 



of the room, tliat it may not condense moisture from the surrounding 

 air. The dryer the atmosphere the better will be the action of the 

 machine. 



35O. Electric Density or Tension. We already 

 have the idea that all bodies, at all times and under all 

 conditions, have a certain quantity of the electric fluid. 

 This quantity may be all -f or all , or partly one and 

 partly the other, the + and mingling in all propor- 

 tions. The kind may vary; the quantity is constant. If 

 this quantity be equally divided between the + and the , 

 the body is unelectrified. If the + be slightly in excess, 

 the body is feebly charged positively, and vice versa. If 

 more be replaced by more +, the positive charge is 

 increased ; that is, the excess of the positive over the neg- 

 ative is increased. This excess is the resultant or available 

 force. If all of the could be removed and an equal 

 amount of -f- substituted therefor, the resultant would 

 equal the total constant quantity and the charge would 

 reach the theoretical maximum. What we call electric 

 density or tension may be considered the value of this 

 excess of either fluid over its opposite. When this excess 

 or difference is great, the charge is said to be powerful or 

 intense ; the density or tension is great. 



QUESTIONS. 



1. (a.) Why is electricity called a force ? (6.) How can you show 

 that it is a force? (c.) Define positive electricity, (d.) Define nega- 

 tive electricity, (e.) How can you show that there are two opposite 

 kinds of electricity ? 



2. (.) How would you test the kind of electricity of an electri- 

 fied body? (&.) Give the theory of two electric fluids. 



3. (a.) What is a proof plane? (&.) An electroscope? (c.) De- 

 scribe one kind of electroscope, (d.) Another kind. 



4. (a.} Define electrics, conductors and insulators. (&.) Show the 



