ELECTRICAL MACHINERY 465 



shows that there is a direct relation between the direction of 

 the current in the wire and the direction of the magnetic 

 lines which encircle it. 



615. Right-hand rule. Ampere devised a rule in which 

 the right hand is used as a means to indicate this rela- 

 tion in all cases. Let the right hand grasp the wire (Fig. 345) 

 so that the thumb points in the direction in which the cur- 

 rent is flowing and the fingers will then point in the direc- 

 tion of the magnetic lines of force. Ampere being the 

 investigator who made quantitative measurements of cur- 

 rent electricity, the unit of measurement was named am- 

 pere in his honor. Owing to the peculiarity of electricity 



it cannot be measured in 

 pints and gills as liquids 

 but can be measured by the 

 chemical effect it will pro- 

 FIG - 345 duce, i.e., one ampere will 



deposit in one second 0.0003286 gram of copper in a copper 



voltmeter. 



616. The ammeter is an instrument used for the 

 measuring of amperes. Commercial ammeters do not 



FIG. 346 AMMETER FIG. 347 VOLTMETER 



measure them by means of chemical deposits, but by 

 means of a needle enclosed in an electrical coil in such a 



