162 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



nometer with a heavy needle (or with a heavy moving coil in the 

 case of the D'Arsonval type of instrument) which is not subject 

 to any perceptible air friction as it vibrates. Such a galvanometer 

 is called a ballistic galvanometer. 



The reduction factor of a ballistic galvanometer may be calcu- 

 lated from the equation 



7/1/X 



k = 



irb 



(60) 



in which / is a known steady current which produces a steady 

 deflection b of the galvanometer, / is the period of one com- 

 plete oscillation of the galvanometer and X is the ratio of two 

 successive throws of the freely swinging needle (or coil) of the 

 galvanometer, swings, the so-called damping ratio of the galva- 

 nometer.* 



87. The flow of current in unclosed circuits. Electrically 

 charged bodies. Consider two insulated metal bodies A and 



B, Fig. IO2#, which at a given 

 instant are connected,as shown, 

 to the terminals of a battery or 

 to any source of electromotive 

 , force. When the wire is con- 

 nected a momentary pulse of 

 current flows through it out of 

 one body and into the other, 

 and the bodies A and B are 

 said to become charged with 

 electricity. 



The body into which the mo- 

 mentary current flows is said to 

 become positively charged and the body out of which the momen- 

 tary current flows is said to become negatively charged, that is, 

 the charge on one body is -j- q and the charge on the other body 



*See Electrical Measurements by Carhart and Patterson, pages 207-213. See 

 Absolute Measurements in Electricity and Magnetism, by Andrew Gray, Vol. II. 

 pages 390-396. See Maxwell's Electricity and Magnetism, Vol. II, pages 374-391, 



Fig. 102a. 



