50 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



and s represents the auxiliary branch. This auxiliary branch 

 is called a s hn tit. 



Example. A galvanometer (or ammeter) of which the resist- 

 ance is R ohms is shunted by a resistance of -/?/99 ohms. In 

 this case 99 times as much current flows through the shunt as 

 through the galvanometer, that is, y^ of the total current flows 

 through the galvanometer and -fa of the total current flows 

 through the shunt. 



28. Use of voltmeter multiplying coils. Suppose one has 

 a voltmeter which is capable of indicating the value of any elec- 

 tromotive force up to a limit of 10 volts (more than 10 volts 

 throws the pointer off the scale, and much more than 10 volts 

 may damage the instrument). Let R be the resistance of the 



Fig. 26a. 



instrument. Let an auxiliary resistance equal to (n i)/? be 

 connected in series with the instrument, let the combination be 

 connected to an electromotive force which is to be measured, and 

 let E be the reading of the instrument ; then the value of the 

 electromotive force is equal to nE. This is evident if we con- 

 sider that a definite deflection on the voltmeter means a definite 

 current flowing through the instrument. Let this current be 

 represented by /. Then the electromotive force between the 

 terminals of the instrument is RI and this is the electromotive 

 force which is indicated by the instrument reading, whereas 

 the electromotive force between the terminals of the combination 

 is equal to the product of the current times the resistance of 

 the combination, or nRI. 



29. Use of a standard shunt and a millivoltmeter, combined, as 

 an ammeter. A millivoltmeter is a voltmeter for reading very 



