476 DIRECT CURRENT UNITS [Ca. XIII 



DIRECT CURRENT UNITS 



655. The Volt. This is the unit of electromotive force, that 

 is the electric force or pressure necessary to produce one ampere of 

 current in a circuit with a resistance of one ohm. 



The difference of potential between the two poles of a Weston 

 standard cadmium cell is i.o 19 volts. The ordinary battery used 

 for ringing door bells has approximately one volt pressure. 



Voltage is a general term representing the pressure in volts in an 

 electric circuit. 



If the difference of pressure between the two given points is 

 great, then the voltage is said to be high; if the difference is slight, 

 then the voltage is low. For example, in projection one might use 

 55 volts for the arc lamp, or 220 volts, or 500 volts. Ordinarily 

 neither the low voltage of 55 nor the high voltage of 500 or 220 

 is used, but an intermediate voltage of no. 



656. The Ampere. This is the unit of current. It is the 

 current which will deposit .001118 gram of metallic silver per 

 second from a 15% solution of silver nitrate in water. It is the 

 current which one volt will maintain in a circuit with one ohm 

 resistance (see below). 



Amperage is the term by which is designated the amount of 



current in amperes flowing at 

 any given moment. If a large 

 amount of current is flowing the 

 amperage is said to be high or 

 great, if a small amount, then it 

 is said to be low or small. For 



example, in projection, the am- 

 FIG. 253. CONNECTIONS OF A VOLT- , , . 



METER TO MEASURE THE ARC perage needed for drawing with 

 VOLTAGE. th e microscope on the house cir- 



V VcTtnSer. cuit ( 493) is small (3-6 am- 



A Arc lamp. peres) , while for opaque pro- 



R Rheostat. jection ( 289), and for moving 



Note that the terminals of the volt- J . ' . fe 



meter are connected to the two points pictures ( 693) in large halls 



between which it is desired to measure t h e amount of amperage needed 



the potential difference. In this case . . . 



it is the two carbons (across the arc), is great (20 to 100 amperes ). 



