HEAT AND LIGHT FROM ELECTRIC CURRENTS 207 



WtATMERPROOf 



magnet, a coil of wire (with an iron core in it) to which a 

 pointer is attached, a spring to return the pointer to zero, 

 and a table of figures or a scale. 



Figure 100 shows how the current enters the ammeter. It 

 enters at the positive terminal and passes into the ammeter 

 through a short conduc- 

 tor (low resistance) to the 

 main line. A small frac- 

 tion of this current goes 

 through a movable coil of 

 wire located between the 

 ends of a magnet. The 

 slight rotation of the coil 

 is magnified, and is regis- 

 tered by the pointer upon 

 the scale. 



The voltmeter differs 

 from the ammeter in two 

 important respects : it is 

 connected in parallel with 

 the main circuit, and it 

 has a coil of wire (resistor) 

 of very high resistance in 

 order that only a small 

 fraction of the current in 



FIG. 101. How electricity is brought into 



the house 

 Note the entrance conduit, the service switch, 



the main circuit may pass the me1 f r > ^ e fuse Pfw and the courses 



followed hy the distributing wires 



through the instrument. 



Electrical instruments, like other machines, vary greatly in size, 

 accuracy, and price. The ammeter should be carefully used 

 and should under no condition be short-circuited across the 

 terminals of any cell or generator. 



These two measuring devices enable us to determine the 

 condition of an electric circuit. The resistance of any con- 

 ductor can be readily found by using Ohm's law. From a 

 wide range of resistance wires one may be selected to carry 



