146 THE HENRY. 



it will be useful to illustrate this relatiou before proceeding further. For 

 this purpose i)erhaps nothing is better than the well-known and oft- 

 repeated comparison of the How of a current of electricity in a conductor 

 to the flow of a stream of water through a. pipe. When water flows 

 from a reservoir through a pipe the quantity which passes any point in 

 the pipe in one second (current strength) depends on tlie height of the res- 

 ervoir above the outlet — that is, on the ''head" or pressure under which 

 it flows — and also ou the resistance which the pipe otters to its motion. 

 The greater the pressure the greater the flow, and the greater the 

 resistance the less the flow. The strength of the current is therefore 

 directly i)roportional to the pressure, and inversely proportional to the 

 resistance. If in this statement "electro-motive" force be substituted 

 for "pressure," it becomes Ohm's law. When these elements are 

 measured in the units given above, the electro motive force in volts, 

 the resistance in ohms, and the current in amperes, the law is expressed 

 very simply by saying that the "current is equal to the electro-motive 

 force divided by the resistance." 



Thus, if the electro-motive force be 1 volt and the resistance of the 

 circuit be 1 ohm the current will be 1 ampere. In an ordinary incan- 

 descent electric lamp the electro-motive force may be about 110 volts, 

 the resistance of the carbon filament when hot about 175 ohms, and the 

 current must therefore be about six-tenths of an ampere. 



The unit of quantity is the coulomb. Charles Augustus Coulomb 

 was a French engineer who made important contributions to science 

 during the latter half of the last century. His character is well shown 

 by the fact that he submitted to imprisonment rather than make a 

 liivorable report upon a i>rop()sed system of canals which he examined 

 as a royal commissioner and which he could not approve. His ingen- 

 ious invention, the torsion balance, enabled him to measure exceedingly 

 small forces with an ac(;uracy hitherto unknown in science; and by its 

 use he made many brilliant researches in electricity, the first in which 

 exact measurement played an important part. A coulomb is the quan- 

 tity of electricity transferred by a current of 1 ampere in one second. 



The unit of capacity is the furaiJ. The name of Faraday might with 

 propriety have attached to more than one unit of electrical measure. 

 His remarkable career — as a newsboy, a bookbinder's ai)prentice, an 

 intensely interested listener to the lectures of Sir Humphry Davy, 

 Davy's helper, later his assistant, and finally his successor at the head 

 of the Eoyal Institution in London — is so generally known that refer- 

 ence to it is hardly necessary. In the history of electricity three 

 splendid discoveries stand incomparably above all others. With the 

 first the names of Galvani and Yolta are associated in the discovery 

 of the new electricity and the means of generating it. In the second. 

 Oersted and Ampere united in laying the foundation of the science of 

 electro magnetism. The third was the discovery of "induction," in 

 which Faraday and Joseph Henry made possible the marvelous develop- 

 ment of the last two decades. 



