Blectrfcal /Measurement. S5 



accomplished by increasing the pressure. So, 

 we can accomplish a similar result in passing 

 electricity through an iron wire at the same 

 rate it flows through a silver wire of the same 

 size, by increasing the pressure, or electromo- 

 tive power; and this is called increasing the 

 voltage. 



The quality of the iron wire that prevents 

 the same amount of current from flowing 

 through it as the silver is called its resistance. 

 The unit of resistance, as mentioned in the last 

 chapter, is called the ohm, and the more ohms 

 there are in a wire as compared with another, 

 the more volts we have to put into the battery 

 to get the same current. 



The unit for measuring the current is called 

 the "ampere," named after the French elec- 

 trician, A. M. Ampere (1789-1836). 



Now, to make practical application of these 

 units. The volt is the potential or pressure 

 of one cell of battery called a standard cell, 

 made in a certain way. The electromotive 

 force of one cell of a Daniell battery is about 

 one volt. One ohm is the resistance offered to 

 the passage of a current having one volt pres- 

 sure by a column of mercury one millimeter in 

 cross-section and 106.3 centimeters in length. 

 Ordinary iron telegraph-wire measures about 

 thirteen ohms to the mile. Now connect our 

 standard coll one volt through one ohm re- 

 sistance and we have a current of one amp* K-. 



