172 APPLIED SCIENCE 



an electrolyte. This process is of great industrial importance. 

 All chemical compounds acids, salts, and bases are 

 made up of two parts; the positive or metallic part, and the 

 negative or non-metallic part. When any compound is dis- 

 solved, it breaks up partially into these two parts. The 

 positive or metallic portion is charged with positive electri- 

 city and is attracted to the negative electrode or plate. 



204. Units of Measurements. A quantity of electricity, 

 like a quantity of water, may be measured. Since the flow 

 or quantity of water depends on the pressure or "head" and 

 on the resistance of the pipes, so the quantity of electricity 

 depends upon the pressure and the resistance of the wires. 



The acting force which gives rise to, or maintains, a current 

 or flow of electricity is called the electromotive force (abbre- 

 viated E.M.F.). The E.M.F. corresponds to pressure in 

 relation to water and is measured by a unit called a volt. 

 That force against which the E.M.F. acts, that is to say, 

 that force which retards the flow or current, is called the 

 resistance, and corresponds to the friction of pipes in rela- 

 tion to water. Resistance is measured by a unit called an 

 ohm. The quantity of electricity corresponds to the quart 

 or gallon of water. The current of electricity, or rate of flow, 

 is measured by a unit called a coulomb, which is the quantity 

 passing per second of time, and corresponds to a flow of 

 water of so many quarts or gallons per second. A rate of 

 flow of one coulomb per second is called an ampere. The 

 unit of rate of electrical work is the product of the E.M.F. 

 and the rate of flow or current just as the pressure with 

 which the force acts is the work performed. The rate of 

 flow of electricity or current is proportional to the impelling 

 pressure or head. 



