CHAP. XIL] ELECTRICITY AND MAGNETISM. 405 



abandoned, the clock being driven by its pendulum, the 

 "bob" of which is an electromagnet alternately attracted 

 from side to side. The difficulty of maintaining a perfectly 

 constant battery current has prevented such clocks from 

 coming into use. 



Electrically controlled clocks, governed by a standard 

 central clock, have proved a more fruitful invention. In 

 these the standard timekeeper is constructed so as to 

 complete a circuit periodically, once every minute or half 

 minute. The transmitted currents set in movement the 

 hands of a system of dials placed at distant points, by 

 causing an electromagnet placed behind each dial to 

 attract an armature, which, acting upon a ratchet wheel 

 by a pawl, causes it to move forward through one tooth 

 at each specified interval, and so carries the hands round 

 at the same rate as those of the standard clock. 



Electric chronographs are used for measuring very small in- 

 tervals of time. A style fixed to the armature of an electro- 

 magnet traces a line upon a piece of paper fixed to a cylinder 

 revolving by clockwork. A current sent through the coils of 

 the electromagnet moves the armature and causes a lateral notch 

 in the line so traced. Two currents are marked by two notches ; 

 and from the interval of space between the two notches the in- 

 terval of time which elapsed between the two currents may be 

 calculated to the ten-thousandth part of a second if the speed 

 of rotation is accurately known. The velocity with which a 

 cannon ball moves along the bore of the cannon can be measured 

 thus. 



434. Electric Telephones. The first attempt to 

 transmit sounds electrically was made in 1852 by Reis, 

 who succeeded in conveying musical tones by an im- 

 perfect telephone. The transmitting part of Reis's 

 telephone consisted of a battery and a contact-breaker, 

 the latter being formed of a stretched membrane, capable 

 of taking up sonorous vibrations, and having attached to 

 it a thin strip of platinum, which, as it vibrated, beat to 

 and fro against the tip of a platinum wire> so making 

 and breaking contact. The receiving part of the instru- 



