162 RECORDS. 



investigation. A gravity contact key devised by Dr. Charles 

 Forbes gives promise of fulfilling the above conditions. Many 

 determinations were made of the times of contact given by the 

 various devices employed on this key, and also investigations 

 carried out on the times of contact of several forms of pendulum. 

 The method employed was as follows : A condenser of 

 known capacity (F farads) was charged during the time of con- 

 tact (T), and the deflection on discharging noted. The con- 

 denser is again charged through a resistance (R) and the de- 

 flection (^Q) observed. 

 Then ; 



Q=EFx {i -c- TjRF) 

 and, 



T=-RFx log ^'(i - QjFF). 



The "gravity key" consists essentially of a rectangular 

 weight falling on metal guides, the key being furnished with a 

 scale divided in fractions of a second, according to the law of 

 falling bodies, and the weight actuating the various forms of 

 switches employed. If two switches are used, one to make the 

 contact and the other to break the contact, by placing them at 

 different distances apart on the scale, times of contact varying 

 from 0.4 second to o.OOi second may be obtained. For shorter 

 times a single switch that makes and breaks the contact is made 

 use of, and the time made faster or slower by placing in different 

 positions on the scale so that the falling weight strikes it with 

 varying velocities. 



In one form the weight moves the short arm of a lever, the 

 long arm passing over a contact strip. Another form is one in 

 which the fulcrum of the lever changes, first giving contact and 

 then breaking the circuit immediately afterwards. In still an- 

 other type the falling weight strikes a lever arm and releases a 

 spring, which makes the contact, and a further motion of the 

 lever breaks the contact, thus giving a differential effect between 

 the velocity of the weight and the rapidity of the spring. With 

 this key it is possible to obtain a contact of only 0.000017 

 second and with careful adjustment it seems possible to reach 

 0.0000 1 second. 



