300 



Mr. C. Hockin on a Lecture-Experiment. 



ing through the corks which stop the tubes, aud then through 

 a stout upright board so as to support the bulbs. Of the thick 

 wires, one from each tube is soldered to one thick copper wire, 

 and the other to another thick wire. These last wires are con- 

 nected with the poles of a battery of one or two large Grove's 

 cells. 



The current then divides itself, and a portion flows through 

 each wire proportional to the con ducting-power of that wire. 

 This current heating the wire causes the air in the bulb to ex- 

 pand and depress the liquid in the tube attached by an amount 

 nearly proportional to the con ducting-power of the wire. If the 

 tubes were filled at first to the top, the liquid will, when the 

 current is flowing, fall through different amounts. Fig. 1 shows 

 the curve when the wires are gold, silver, and various alloys of 

 gold and silver. If the connexions are changed so that the 

 same current flows in succession through every wire, the liquid 

 will be depressed by an amount nearly proportional to the resist- 

 ance of each wire. 



In fig. 2 we have a similar set of bulbs, into each of which 



Fig. 2. 

 (Scale one-tenth of the natural size.) 



passes, through a cork, a bar of metal of the same material as the 

 wire in the corresponding bulb, fig. 1. 



The metal bars are at their other end secured in a box of 

 boiling water. The heat conducted by the bars from the box of 

 water to the bulb depresses the liquid in the tubes, and a curve 

 is formed by the tops of the columns of liquid of nearly the 

 same shape as was formed by the heated wires. 



It is well to protect the bulbs in both cases by enclosing them 

 in little compartments formed of blackened tin-plate. These 

 may be conveniently formed by soldering pieces to form the ver- 



