ELECTRICAL INVENTIONS 



227 



and the water outflows from the latter with a force equal to the 

 sum of the pressures. If batteries are connected parallel, the 

 effect is similar to connecting a small pipe running from each of 

 several water tanks standing at the same level with one large 

 pipe (Fig. 94). The combined outflow is greater, but the pres- 

 sure in the large pipe is no greater than it is in a small pipe run- 

 ning from one tank. Stated in electrical terms, we say that when 

 batteries are connected in series the current has a voltage equal 

 to the combined voltages of the several batteries, but the amper- 

 age is no greater than that of one of the batteries. When con- 

 nected in parallel, the amperage is increased while the voltage 

 remains the same. 



Resistance is measured in ohms. The ohm is about the 

 resistance offered by 9.3 feet of No. 30, American gauge copper 

 wire. To overcome high re- 

 sistance, high electric pressure 

 must be used. Electric pres- 

 sure is expressed in terms of 

 volts. Just as with liquids, 

 so with electric currents, the 

 greater the pressure, the 

 greater the flow, other things 

 being equal. The unit that 

 is used in measuring the rate 

 of flow of electricity is the 

 ampere. It is denned as that amount of current which, while flow- 

 ing through a standard solution of silver nitrate, such as is used in 

 silver plating, will deposit a specified amount of silver (0.001118 

 grams) per second. The electric pressure that will force a cur- 

 rent of one ampere through a resistance of one ohm is desig- 

 nated the volt. 



The instrument used for measuring the amount of current 

 flowing in a wire at any minute is called the ammeter (Fig. 95). 

 A soft iron core wound with insulated wire is pivoted at its mid- 

 point, and so mounted between the ends of a permanent magnet 



FIG. 95. Diagram of an ammeter 



