296 



The Rev. T. E. Eobinson's Experimental Researches on the 



The helices are made of lapped copper wire, No. 12, or ^ inch diameter, 

 coiled in four layers on mahogany bobbins, 2§ diameter, and 10"9 long. The 

 two have 638 spires, and 483 feet of wire ; each layer being well soaked with 

 lac varnish. I used wood for these bobbins, to prevent the magnet from being 

 much heated when powerful currents are employed, but in all subsequent 

 helices used copper, as the wires were sometimes so hot that I feared for their 

 covering.* The external diameter of the helices is 33 inches. 



2. The weighing apparatus is shown in the wood-cut. It consists of a strong 



oak table, T, 32 by 16 inches, and 2 inches thick, in which are, inlaid and se- 

 cured by strong wood screws, two pieces of § boiler-plate. On one of these is 

 fixed the magnet by a strong bolt tapped into the centre of its base B, and set 



* On one occasion, with metal bobbins, the magnet and its keeper were heated 35° in 70 minutes. 



