234 Description of some Lecture-experiments in Electricity. 



B represents a galvanic battery of one 

 or two cells, K a key for making and 

 breaking the battery circuit, G the gal- 

 vanometer, C and C two coiled conduc- 

 tors, with or without iron cores, and R 

 and R' two zigzag or uncoiled conduc- 

 tors, of which the resistances are so ad- 

 justed relatively to the resistances of C 

 and C that, when the battery-contact 

 is permanently maintained, no current 

 passes through the galvanometer. Then, 

 on completing the circuit, there is a 

 temporary deflection of the galvanome- 

 ter due to the inverse extra-current, 

 and on breaking it there is an opposite 

 deflection due to the direct extra- cur- 

 rent. The reason of this is easily seen. 

 Supposing p to be the positive and n 

 the negative pole of the battery, when 

 the key K is pressed down the current 

 is immediately established in the circuit 



BRfliR'B, causing a corresponding deflection of the galvano- 

 meter ; after a very short interval, however, the current is also 

 established in the circuit B C b a C B, and brings the galvano- 

 meter-needle to rest. On raising the key the current ceases in- 

 stantaneously in the uncoiled conductors R and R y , but continues 

 for a short time in the coiled conductors C and C, traversing 

 the galvanometer from b to a and causing a momentary deflec- 

 tion in the opposite direction to that produced on making the 

 battery-circuit. Using for the conductors C and C the primary 

 wire of a medium-sized Ladd's induction-coil and the wire of a 

 straight electromagnet, and uncoiled German-silver wires for 

 the conductors R and R', I obtained with one cell of Grove's bat- 

 tery a swing of from 50° to 60° on a large astatic galvanometer 

 with heavy needles 8 inches long on completing the battery-cir- 

 cuit, and an equal swing in the opposite direction on breaking- 

 contact after the needles had come to rest. The directions of 

 the swings were such as to indicate that the current both com- 

 menced and ceased more suddenly in the uncoiled than in the 

 coiled conductors. 



The only special precaution that need be pointed out in order 

 to ensure the success of this experiment, is that the resistances 

 of the several conductors shall be so small, and their mass so 

 great, that they may'- not become sensibly heated and so have 

 their relative resistances changed during the passage of the 

 current. 



