RESISTANCE AND ELECTROMOTIVE FORCE. 51 



small electromotive forces, and it is called a millivoltmeter because 

 its scale reading indicates the value of an electromotive force in 

 millivolts (one millivolt equals one one-thousandth of a volt). 

 The current / to be measured flows through a known low resist- 

 ance R, and the electromotive force between the terminals of 

 this resistance is measured by means of a millivoltmeter as indi- 

 cated in Fig. 26$. If the value of R is one one-thousandth of an 

 ohm, then the reading of the millivoltmeter in millivolts is the 

 value of the current in amperes. If the value of R is one one- 

 hundredth of an ohm, then the reading of the millivoltmeter in 

 millivolts must be divided by 10 to give the value of the current 

 in amperes. If the value of R is one tenth of an ohm, then 

 the reading of the millivoltmeter in millivolts must be divided by 

 100 to give the value of the current in amperes. It is evident 

 from the connections shown in Fig. 26b that the total current is 

 equal to the current in the known resistance R plus the current 

 flowing through the millivoltmeter ; but inasmuch as the resist- 

 ance of the millivoltmeter is always quite large, the current which 

 flows through it is very small and is always negligible in com- 

 parison with the current which flows through R. The resistance 

 R in Fig. 26b forms a shunt to the millivoltmeter and the combi- 

 nation exemplifies the matter which is discussed in Art. 27. 



PROBLEMS. 



13. A current of 0.5 ampere flowing through a glow lamp 

 generates 150 calories of heat in 10 seconds, (a) Required the 

 resistance of the lamp in ohms. (&} What power is expended in 

 the lamp ? Express in watts and in horse-power. Ans. (a) 252 

 ohms ; () 63 watts or 0.0844 horse-power. 



14. A wire having a resistance of 250 ohms is coiled in a 

 vessel containing 2,000 grams of oil of which the specific heat is 

 0.60. The vessel itself weighs 200 grams and its specific heat 

 is 0.095. A current of 1.5 amperes is passed through the coil of 

 wire. How long will it take to raise the temperature of the oil 

 and the vessel one centigrade degree ? Ans. 9. 1 1 seconds. 



