472 proceedings of the american academy. 



Description of Apparatus used in Later Experiments. 



The Storage Battery. 



The storage battery which charged my condensers is of the same 

 type as the large 40,000 volt battery used by Professor Trowbridge for 

 discharge experiments in tubes of high vacua, although it has a total 

 voltage of only about 900 volts. The cells are test-tubes with lead 

 strips dipping in a sulphuric acid solution ; they are placed in racks 

 of paraffined whitewood, each rack holding two rows of 30 cells each. 

 Such a storage battery cannot yield large steady currents for any con- 

 siderable time, but for furnishing a constant electromotive force and 

 for charging condensers it is extremely useful. An hour or two of 

 charging the battery early in the morning is usually sufficient to give 

 it a fairly constant voltage for the whole day. 



The Air Condenser. 



The preliminary experiments briefly described above, although 

 quantitatively almost worthless, showed clearly two things : first, 

 that residual charges can form in considerable amounts in a very short 

 time interval, say in a tenth of a second ; and, secondly, that if the 

 neutralizing two-condenser method was to yield the best results, in 

 fact if it was to yield results of any quantitative value at all, it would 

 be necessary to construct a standard condenser which should be free 

 from residual charge formation, or which should show this effect only 

 to a negligible degree. I therefore decided to build an air condenser 

 of such capacity that its charge might give ballistic throws of large 

 amplitudes, so that the "difference effect," when used against a test 

 condenser in the manner already described, might still be of measurable 

 magnitude. An air condenser was desirable because gaseous dielectrics, 

 if they form residual charges at all, do so only in exceedingly minute 

 quantities. 



I selected, therefore, twelve large sheets of very flat plate-glass from 

 the stock of the Boston Plate and Window Glass Co. in South Boston. 

 Of these, seven were of dimensions 63.5 by 66 cms., and the other five 

 were 61 by 6(5 cms. Their thicknesses varied considerably, being from 

 0.8 to l.o cm., but this did not make any difference for my purpose. 

 The plates were carefully cleaned, and then on both sides of each 

 plate tinfoil sheets were pasted with Higgins' Photo-Mounter paste 

 considerably softened with water. It was found that the best results 

 could be got when a squeegee roller, continually dipped in water, was 

 used to roll out the tinfoil sheets and to force out all the surplus 



