20 



DESCRIPTIVE CATALOGUE. 



broad surface of the leaf becoming convex towards the 

 magnet in the one case, and concave in the other. The 

 curvature of the gold leaf may be viewed in a direction at 

 right angles to the line of its motion, and may be referred 

 to a fine line drawn upon the tube in the direction of its 

 axis. This instrument is a most delicate test of the exist- 

 ence and direction of a weak galvanic current. 



Price, with magnet, $5.00. 



Fig. 437. 



Fig. 438. 



Barlow's Electro- Magnetic Glole (Fig. 437) is a globe of 

 wood or pasteboard, supported upon two wooden uprights. 

 The globe has a wire, covered with silk or cotton, coiled 

 completely over it, from one pole round and round to the 

 opposite pole. One end dips into the cup, p ; the other into 

 the cup, N. When the two poles or i'ons of a galvanic 

 battery are connected with the wires in p and N, the fluid 

 circulates around the globe, in consequence of which the 

 globe becomes magnetic, though there is no iron attached 

 to it ; and a small delicately-balanced magnet placed upon 

 any part of it will indicate the polarity and dip, in exactly 

 the same manner as the usual magnet does upon the earth 

 itself. Price, $3.00. 



Marsh's Vibrating Wire. (Fig. 438.) This arrangement 

 consists of a mahogany base, supporting a brass pillar bent 

 over at the top, and having a screw cup entering the end, 

 in the top of which mercury may be put ; in the base is a 

 trough for mercury, and a mercury cup at the side, sup- 

 ported by a wire connected with the trough ; to the end of 

 the screw cup at the top there is a slender wire suspended 



