194 PROCEEDINGS OP THE AMERICAN ACADEMY 



measured by a large tangent galvanometer was approximately constant 

 during the whole series of experiments. 



This apparatus was placed in a basement about sixty feet from the 

 testing-table on the floor above. A pair of carefully insulated wires 

 made connection between the condenser and the testing-table. On 

 the table was a Thomson reflecting galvanometer of 7,000 ohms re- 

 sistance, and the charging key. A water battery was used to charge 

 the condensers, and the charging potential varied at need from 3 to 

 175 volts. 



The I- microfarad condenser was charged and discharged through a 

 plain key, the magnet being alternately excited and inactive. 



Encouraging but not decisive indications were obtained. The varia- 

 tion was so small a jiart of the total capacity, that it was masked by 

 errors of experiment and observation. 



To reduce the deflection of the needle to a manageable amount 

 and yet retain the same variation, if any, a new condenser of about the 

 same capacity was made, and three keys used. By the first, both con- 

 densers were charged simultaneously to the same potential ; by the 

 second, they were discharged into each other; and by the third, the 

 difference between their charges was discharged through the galva- 

 nometer. 



The use of three keys was found to introduce great errors, arising 

 from variations in the time given to each of the operations mentioned 

 above, and a new key was made which required but a single motion in- 

 stead of three. After adopting a uniform time of charging — in the 

 majority of cases fifteen seconds — fairly constant results were obtained, 

 and some of the earlier observations reported are taken from experi- 

 ments in which this key was used. Ultimately on account of objections 

 to the length of time of charging, as well as to secure greater uniformity 

 in the separate periods allotted to each part of the process, a new key 

 was constructed, which proved very satisfactory in use, and which has 

 been employed exclusively since that time. It consisted of a heavy 

 |-second pendulum furnished with adjustable catches, by whose use it 

 could be made to execute a single vibration through any required 

 amplitude, and could be held suspended at the end of its swing. 



The pendulum carried on its under side two insulated metal pieces, 

 from each of which depended three small platinum wires. 



These wires in the course of their vibration cut through drops of 

 mercury connected with the poles of the battery, the poles of the con- 

 densers, the binding-screws of the galvanometer, and each other, in 

 such a manner as to produce the desired connections. 



