Chap. iv. j INDUCTION. 35 



Let E (Fig. 19) be an element from the positive pole 

 of which the wire A passes to a cup containing 

 mercury, into which it dips at B. Let c be the wire 

 from the negative pole also dipping into the mercury. 

 Let FD be another wire running parallel to AB, and in 

 its neighbourhood, and let the ends of FD be connected 

 with a galvanometer, an instrument for indicating the 

 presence of currents of electricity by the movement of 

 a magnetic needle. It is fully 

 described in chapter x. We 

 have thus two circuits, the one 

 in connection with the cell, which 

 may be called the cell or battery 

 circuit, or better, the PRIMARY 

 CIRCUIT ; the other, in connection 

 with the galvanometer, is the 



T . , Fig. 19.-Scheme of lu- 



galvanometer circuit, or SECON- duction. 



DARY CIRCUIT. When one of 

 the wires coming from E is withdrawn from the 

 mercury cup the circuit of the cell is opened, and the 

 electricity ceases to flow. When the wire is re-immersed 

 in the mercury the circuit is closed, and the current is 

 re-established. 1. Now supposing the battery circuit 

 to be open, let it be closed by dipping the wire into 

 the mercury ; at the moment of re-establishing the 

 current a current appears in the galvanometer cir- 

 cuit. It lasts for a very short time and then disap- 

 pears, provided no change takes place in the primary 

 circuit. 2. Supposing, next, the primary current to 

 be flowing steadily, let it be interrupted by removing one 

 of the electrodes from the mercury ; at that moment 

 a current appears in the secondary circuit, as indi- 

 cated by the swing of the magnetic needle. It lasts 

 also a very short time and then disappears. This 

 phenomenon is called INDUCTION ; the primary current 

 is the inducing, and the secondary the induced, 

 current. 3. If the primary current be flowing, 



