Lemon.— Ow Duplex Telegraphy. 397 



hence there is no current moving in z w, for the potential is equal at the points 

 z and w in either circuit ; a galvanometer or relay placed in this cross 

 connection would show no sign of a current passing, for in both cases the 

 currents will have equal and opposite effects on the instruments so placed in 

 the cross connection. This condition will hold good even though the resistance 

 of one circuit be double that of the other, for at definite proportional points 

 equal potentials exist, and. no currents can therefore pass from one point to 

 the other in the branch connection. 



In making the foregoing statement, I wish it to be understood that, although 

 no current passes from ;^ to w or from w to z when the currents are circulating 

 in the two circuits A B and C D, still I am of the opinion that the coils of the 

 instrument placed in the cross connection z w are charged from either end at 

 the same instant of time, and remain so charged so long as the current is 

 circulating in both the circuits of A B and C D ; and at the moment the current 

 ceases to circulate in both circuits (which will be at one and the same time), 

 the charges that have rushed in from either side make their exit the way they 

 entered, and through their having the same potential and the same velocity, 

 any effect the outgoing current on the one side might have on the instrument 

 so placed in the cross connection is counteracted by the outgoing current on 

 the other side. This can be easily proved by placing a galvanometer exactly 

 in the middle of a circuit, and sending the same current, equal as regards 

 quantity and potential, from either end. When the one current is sent, the 

 needle lies over to the one side, when the other current is sent the needle 

 lies over to the other side j but when both currents are sent on the line 

 at one and the same time the needle keeps its normal or upright position, for 

 the simple reason that both currents are alike in their potential, and con- 

 sequently have equal and opposite effects on the needle, or, in other words, the 

 two forces being equal, they apparently neutralize the effects of each other. 

 But let the potential of one current (the quantity on both sides being still 

 the same) be more than the other, then the effect of the strongest current will 

 be just weakened by an amount equal to the potential of the opposite and 

 weaker current, for the pressure on the one side will be greater than the 

 other. It is by being able to balance these potentials at two definite pro- 

 portional points opposite to each other, that duplex telegraphy on the 

 Wheatstone Bridge principle is made practicable. 



The term " current," I would here remark, is only used for convenience 

 sake. What electricity is, is not known as yet. Some writers seem to infer 

 that it is another form of heat. All writers agree, however, in treating it as 

 a force ; and in its utilization, by applying the laws that govern forces, results 

 looked for can always be obtained, provided due care is taken as regards its 

 peculiar nature whilst conducting experiments by its agency. 



