PLEASANT WAYS IN SCIENCE. 



FIG. 5. 



graphy, this state of things is altered, the needle at the 

 sending station being left unaffected by the transmitted 

 current, so as to be able to receive messages, and in self- 

 recording systems to record them. This is done by dividing 

 the current from the battery into two parts of equal 

 efficiency, acting on the needle at the transmitting station in 

 contrary directions, so that this needle remains unaffected, 

 and ready to indicate signals from 

 the distant station. The principle 

 of this arrangement is indicated in 

 Fig. 5. Here a b n represents the 

 main wire of communication with 

 the distant station, coiled round 

 the needle of the transmitting 

 station in one direction > the dotted 

 lines indicate a finer short wire, 

 coiled round the needle in a con- 

 trary direction. When a message 

 is transmitted, the current along the main wire tends to 

 deflect the needle at n in one direction, while the current 

 along the auxiliary wire tends to deflect it in the other 

 direction. If the thickness and length of the short wire are 

 such as to make these two tendencies equal, the needle 

 remains at rest, while a message is transmitted to the distant 

 statior along the main wire. In this state of things, if a 

 current is sent from the distant station along the wire in the 

 direction indicated by the dotted arrow, this current also 

 circuits the auxiliary wire, but in the direction indicated by 

 the arrows on the dotted curve, which is the same direction 

 in which it circuits the main wire. Thus the needle is 

 deflected, and a signal received. When the direction of the 

 chief current at the transmitting station is reversed, so also 

 is the direction of the artificial current, so that again the 

 needle is. balanced. Similarly, if the direction of the current 

 from the distant station is reversed, so also is the direction 

 in which this current traverses the auxiliary wire, so that 

 again both effects conspire to deflect the needle. 



