■1S2 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



to reach the edge of its lead strip. While it cuts through this, the 

 north knife still ploughing across its surface of lead, the potential dif- 

 ference of the test condenser is made zero. This short-circuiting lasts 

 for, perhaps, 0.001 of a second or more, if a knife edge notches the 

 whole edge of a lead strip, but may be as short as 0.00007 of a second, 

 when a knife point barely notches the sharp edge of a lead strip which 

 has been filed down to a narrow V-point. After the iron weight has 

 been dropped from its trigger device and has thus charged and short- 

 circuited the test condenser momentarily, a brief time is allowed the 

 condenser for the residual charge to become " free," and then it is dis- 

 charged through the d'Arsonval galvanometer. 



Figure 3. 



The results obtained by these experiments are not of much quantita- 

 tive value ; for there is no way of knowing how much of the residual 

 charge discharges during the short circuit along with the " free charge." 

 What residual charge can form in 0.0032 of a second, which is the 

 usual charging time in these experiments, is necessarily of a very 

 mobile character, and perhaps a large part of it discharges in a short 

 circuit even as brief as 0.00007 of a second. There is thus no reason 

 to expect a number of measurements, taken under apparently the same 

 conditions, to agree very closely ; for a very slight difference in the 

 time of short-circuiting may, perhaps, cause a large difference in the 

 residual charge remaining behind. 



As remarked above, the usual charging time in these experiments, or, 

 more accurately, the time in which the test condenser is under the 



