102 SEC. 3. MEASUREMENT. 



The measurement of the velocity of projectiles is effected by the passage of 

 an electric spark at the moment when the projectile touches an insulated wire 

 which reaches to the inside of the gun, and thus is free from retardation 

 caused by the inertia of matter or magnetism. 



The same apparatus can be used for the measurement of the velocity of 

 electricity in suspended wires. 



The complete apparatus comprises a Leydeu jar, induction coil, commutator, 

 gun barrel, chronograph, and two batteries. 



407c. Recording Cylinder, with original marks by which 

 the speed of electricity in iron wires has been measured. 



Dr. Werner Siemens. 



Those marks which are surrounded by a halo or circle indicate the com- 

 mencement of the discharge, the successive series of small marks has been 

 formed by the electricity which has traversed the conductor ; the angular 

 distance between the first-mentioned mark and the first point of the series 

 gives the measurement of the speed of electricity. By measurement of the 

 time which the electricity requires to pass through lines of various lengths, 

 the electrostatic retardation, which is proportional to the square of the length 

 of the line, has been eliminated. By these researches it has been shown 

 that electricity is transmitted in conductors with a constant velocity which is 

 independent of the static retardation, and which for iron amounts to about 

 230,000 kilometres per second. 



(See Monatsberichte der Kgl. Pr. Acad. der Wissenschaften, 6 Dec. 1875.) 



411. Complete Apparatus for measuring the Velocity 

 of Projectiles in the bore of a gun, and for measuring the speed 

 of electricity. Siemens and Halske. 



412. Vibration Chronograph, for measuring the time of 

 descent on an inclined plane, executed according to Beetz, by 

 M. Th. Edelmann, at Munich. (A description accompanies the 

 object.) Prof. Dr. Beetz, Munich. 



413. Edelmann's Apparatus for the descent of a fall- 

 ing Body accessory to Beetz's chronograph. 



M. Th. Edelmann, Munich. 



413a. Chrono-Gonioxneter, with magnifier. 



Le Vicomte Duprat, Consul- General of Portugal. 



D. STROPHOMEJERS. 

 44. Counters and Speed Indicators. 



T. R. Harding and Son. 



(a.) Counters with reciprocating motion, as applied to marine and 

 stationary engines. 



Counters with rotary motion, suitable for shafting, printing, and other 

 machinery. 



Small counters with rotary motion applicable to spinning machinery and 

 various other purposes. 



Pocket counters for ascertaining the speed per minute of spindles or quick 

 running machinery up to 10,000 revolutions per minute. 



(6.) Counters actuated by pneumatic and electric apparatus at a distance 

 from the motion to be indicated. 



(c.) Speed indicators, showing by the height of a column of mercury the 

 actual speed, at any moment, of engines and other machinery. 



