84 Nature's flMractes. 



area of a cross-section of the iron wire must 

 be over seven times that of the silver wire. 

 But if we want to keep both wires the same 

 size and still force the same amount of cur- 

 rent through each we must increase the pres- 

 sure of the battery connected with the iron 

 wire. We measure this pressure by a unit 

 called the "volt," named for Volta, the in- 

 ventor or discoverer of the voltaic battery. 

 The volt is the unit of pressure or electro- 

 motive force. (In all these 1 cases a "unit" is 

 a certain amount or quantity as of resistance, 

 electromotive force, etc. fixed upon as a 

 standard for measuring other amounts of the 

 same kind.) 



The iron wire offers a resistance that is 

 about seven times greater than silver to the 

 passage of the current. To illustrate by water 

 pressure: If we should have two columns of 

 water, and a hole at the bottom of each column, 

 one of them seven times larger than the other, 

 the water would run out much faster from the 

 larger hole if the columns were the same 

 height. Now, if we keep the column with the 

 larger hole at a fixed height a certain amount 

 of water will flow through per second. If we 

 raise the height of the column having the 

 small hole we shall reach a point after a time 

 when there will be as much water flow through 

 the small hole per second as there is flowing 

 through the large hole. This result has been 



