96 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



dure is susceptible of great accuracy, and that it furnishes satisfactory 

 means of determining the velocity of a rifle ball, or the rate of propa- 

 gation of an elastic wave in a bar of metal, or the time of contact of a 

 hammer and an anvil. Some time ago I had to measure with some care 

 an interval of time of about one five thousandth of a second, and to 

 test the apparatus, used a good number of different combinations of 

 condensers and resistances in the circuit, with the result that the 

 determinations agreed within the very small uncertainty caused by 

 the fact that it was difficult to be sure that the suspended system 

 of the ballistic galvanometer used was originally exactly at rest. 



The arrangement of the apparatus for measuring the time of closure 

 of the key is shown schemetically in the subjoined diagram, ^repre- 

 sents a divided mica condenser of about 28.8 microfarads capacity; 



G, a ballistic mirror galvanometer ; R, a large known resistance ; T is 

 the point of contact of the key with its base, or of the thimble with its 

 block ; P and Q are keys mounted on a large block of pai-rafine for 

 discharging the condenser and short-circuiting the galvanometer after 

 every throw. In order to make the potential difference across the open 

 key at all times small, the ends of the condenser circuit were attached 

 to two points {X, Z) in the closed high resistance current of one or 

 two cells, so chosen as to make the galvanometer throw, corresponding 

 to a complete loading of the condenser, a convenient number of centi- 

 meters ; the final adjustment of this throw to a predetermined value 

 was made by moving the scale (along a track made for the purpose) 

 away from or nearer to the galvanometer mirror. 



It is evident that if k is the capacity in farads of the condenser, 

 r the whole resistance in ohms of the condenser branch circuit, and 

 (u -f v), of which V is the resistance of the plain conductor between 

 A" and Z, in the resistance of the battery circuit, then, when F and Q 



