200 ELECTROMETRY. 



brought more or less close to the knob A, connected to the inner 

 coating. If the knob B is connected with the earth, and the knob A 

 with the source of electricity, a spark passes between the two knobs 

 when the potential of the inner coating is that corresponding to the 

 striking distance, and all the sparks allow the same quantity of 

 electricity to pass. 



Lane's jar is particularly used in measuring the charge of a 

 battery ; the experiment may be arranged in two ways. 



The battery being insulated, the inner coating is connected with 

 the source of electricity, the outer coating with the knob A of 

 Lane's jar, and the knob B with the earth. Let C be the capacity 

 of the battery, c that of Lane's jar, V the potential of the inner 

 coating of the battery, v that which remains on the outer coating at 

 the end of the experiment, and n the number of sparks observed. 

 The charge M of the battery is M = C (V - v). As this battery forms 

 a closed condenser, the outer coating has a charge equal to -M + m, 

 the charge m being that which produces the potential v. The total 

 quantity of electricity which has passed through the Leyden jar being 

 M - m, if q is that which corresponds to each spark, and Vj is the 

 difference of potential at the striking distance, we have 



M - m =Q3,L GORSf* m + nq. 



The product nq, or ncV, only represents the charge of the battery 

 provided the quantity m may be neglected, as is usually the case. 



Most frequently Lane's jar is insulated ; the knob A is connected 

 with the source, and the knob B with the inner coating of the battery, 

 the outer being to earth. As the production of the spark between 

 two conductors only depends on the difference of potentials, and not 

 on their absolute values, the same quantity of electricity passes at 

 each spark ; in this case the charge of the battery is exactly propor- 

 tional to the number of sparks. 



When the charges to be measured are feeble, Lane's jar may be 

 replaced by Gaugain's discharge electrometer (Fig. 159), which plays 

 exactly the same part. This is a gold-leaf electroscope, provided 

 with a knob in connection with the earth, which is placed in the 

 plane of divergence of the leaves, and within reach of one of them ; 

 when the charge is sufficient, contact takes place, and the instrument 

 is discharged. 



If electricity flows out continuously, a succession of perfectly 

 identical discharges is produced, provided the gold-leaf falls down 

 again immediately. When the charge of an electrified body is to be 



