45, COBNHILL, B.C., AND 122, BEGENT STEEET, W,, LONDON. 449 



FIG. 2714. FIG. 2715. 



Each. Each. 



B. d. s. d. 



2714 Model Electro-Magnetic Telegraph, fitted with 



reversing brake, of a simple form, on a stand, suited 



for Lectures or the Class-room (fig. 2714) ... 330 



2715 Model Electro -Magnetic Signal Bell, for use in 



conjunction with above (fig. 2715) 1 12 



2716 Electro -Magnetic Bells, for Household purposes, &c., see page 461. 



2717 Copper Wire, covered with Cotton, superior quality, in 



long lengths. The following prices are variable : 



Nos. 12 to 14. 15 to 18. 19 to 22. 23 to 26. 30 to 32. 



Perlb. 3s. 4s. 5s. 6s. 8s. 6d. 



2718 Copper Wire, covered with Silk, to order, per lb., variable. 



2719 Ditto ditto covered with Gutta Percha, in lengths of 



100 feet and upwards of various sizes, from, per length 110 



2720 Assortment of Electro-Magnetic Apparatus, packed 

 in a strong Pine Case, to illustrate the first principles 

 of the science 330 550 



These sets are so arranged as to extend and complete those of Galvanic Apparatus, page 436. 



Of the terms POSITIVE and NEGATIVE. There is nothing which has a greater 

 tendency to confuse the mind, with regard to voltaic apparatus, than the terms 

 positive and negative end of a battery. " The fundamental principle," observes 

 Mr. Walker, " which cannot be too strongly enforced, is that the passage of the 

 electricity is from the zinc to the copper." This, of course, refers to the common 

 forms of battery Cruikshank's, Babington's, &c. In the arrangement of Smee, 

 the passage of the electricity is from the zinc to the silver; in Grove's battery, 

 from the zinc to the platinum. " The positive is the end where the electricity 

 leaves the battery : the negative where it re-enters it. The direction taken by the 

 current being ascertained by the mere inspection of the situations of the two metals 

 in a cell, the other points follow as a necessary consequence." Now, taking the 

 Smee's battery as an illustration, it must be clear, that as the electricity passes 

 from the zinc to the silver, it would leave the battery by the wire attached to the 

 silver plate, and having passed through the interposed apparatus, would return to 

 the battery by the wire attached to the zinc plate ; the silver, which is the negative 

 metal, forming the positive end of the battery ; and the zinc, the positive metal, 

 forming the negative end. In like manner with all the batteries we have described, 

 the zinc, though the positive metal, is the negative pole. 



2 G 



