206 ELEMENTS OF GENERAL SCIENCE 



closed no water can flow, and if the electric switch is open 

 there can be no flow of the current. The nature of the path, 

 or the resistance, has a great influence on the current. The 

 greater the resistance, the less the current, and vice versa. 

 The unit for measuring this resistance is called the ohm in 

 honor of Georg Simon Ohm (1787-1854), a noted German 

 physicist. The ohm is approximately the resistance of 160 

 feet of No. 18 copper wire of the variety used for lamp cords. 

 Other values are shown in the table on page 204. 



All these facts are summarized in the following law, which 

 was announced by Ohm in 1826, this law showing the rela- 

 tions between current (/), electromotive force (JT), and resist- 

 ance (-R). The values for the resistances in the last column 

 of the table on page 204 were calculated by means of the 

 second equation. 



OHM'S LAW 



electromotive force 

 Current = - 

 resistance 



volts E 



current 



volts E 



Ohms = - R = 



amperes / 



Electromotive force = current x resistance 



Volts = amperes x ohms E = IR 



This formula is used extensively in all electrical work. 



219. Practical uses of voltmeters and ammeters. The volt- 

 meter and ammeter are as necessary in the use of electricity 

 as scales are in the grocery business. The instruments shown 

 in figure 100 are modified forms of voltmeters and ammeters 

 designed for automobile service. 1 They consist of a U-shaped 



1 Large charts illustrating ammeters and voltmeters may usually be 

 obtained free from supply companies such as the Weston Electrical 

 Instrument Company, Newark, N.J. 



