viii CONTENTS. 







PAGE 



current which arrives at a station. 119. To make it ring the 

 alarum. 120. Station with two alarums. 121. Notice of the 

 station transmitting and receiving signals. 122. When signals 

 not addressed to the station the current is passed on. 123. How 

 to receive a despatch at the station, and stop its farther progress. 

 124. How several despatches may be at the same time sent 

 between various stations on the same line. 125. Secondary lines 

 of wii-e then used. 126. Recapitulation. 127. Signals by com- 

 binations of unequal intervals of transmission and suspension. 

 128. Key commutator. 129. Horological commutator for a 

 current having equal and regular pulsations. 130. Case in which 

 the pulsations are not continuous or regular. 131. No limit to 

 the celerity of the pulsations. 132. Application of a toothed 

 wheel to produce the pulsations. 133. By a sinuous wheel. 

 134. Method of diverting the current by a short circuit, its 

 application to the alarum. 135. Effects of the current which 

 has been used for signals. 136. Deflection of magnetic needle. 177 



CHAP. VI. 137. Relation of the deflection to the direction of 

 the current. 138. Galvanometer or multiplier. 139. Method 

 of covering the wire. 140. Method of mounting the needle. 

 141. Method of transmitting signals by the galvanometer. 142. 

 How the current may produce a temporary magnet. 143. 

 Electro-magnet constructed by Pouillet. 144. Electro-magnets 

 formed by two straight bars. 145. They acquire and lose 

 their magnetism instantaneously. 146. Magnetic pulsations as 

 rapid as those of the current. 147. How they are rendered 

 visible and counted. 148. Extraordinary celerity of the oscil- 

 lations thus produced. 149. They produce musical sounds by 

 which the rate of vibration may be estimated. 150. How 

 the vibrations may impart motion to clock-work. 151. Their 

 action on an escapement. 1 52 . How the movement of one clock may 

 be transmitted by the current to another. 153. Hew an electro- 

 magnet may produce written characters on paper at a distant 

 station. 154. How the motion of the hand upon a dial at one 

 station can produce a like motion of a hand upon a dial at a 

 distant station. 155. How an agent at one station can ring an 

 alarum at another station. 156. Or may discharge a gun or 

 cannon there. 157. Power of the bell or other signal not 

 dependent on the force of the current. 158. Mechanism of 

 telegraphic alarum. 159. Various alarums in telegraphic oifices. 

 160. Magneto-electricity. 161. Method of producing a 

 momentary magneto-electric current. 162. Application of an 

 electro-magnet to produce it 193 



