4SS 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



horizontally by one of the south holders (s, upper one), while at the 

 same time one of the high voltage east knives ploughed over the sur- 

 face of its lead strips, shown at e. The test condenser c could be 

 charged either by means of the north knife, which gives from one to two 

 centimeters of ploughing contact, or by means of lead strips placed in 

 the other south lead clamp. The time of charging could here be varied 

 by letting a knife point notch the edge of one, two, or three thick- 

 nesses of lead strips (s, lower strip), placed together with their edges 

 all even, or by letting the sharp knife point barely dent the sharpened 

 edge of a single lead strip, as in the short-circuiting experiments. By 



Figure 4. 



the use of very fine knife points and very sharp edges of the lead strip 

 it was estimated that charging times as short as 0.00005 of a second 

 could be obtained, if the lead strip was carefully adjusted so that the 

 knife point would just slightly notch the sharpened edge. More often 

 the time would be about 0.00007 of a second, and this number is 

 usually taken in reducing the observations. Each thickness of lead 

 strip adds 0.00012 second to the charging time, but the number 

 0.00020 has been adopted as the charging interval when the knife 

 point notches the whole edge (0.7 mm.) of a single lead strip, because 

 in this case the strip was not adjusted to be notched on quite so 

 narrow a margin. The height above these lead strips, from which the 

 iron weight with the knives was usually dropped and for which the 



