1C6 ANNUAL OF SCIENTIFIC DISCOVERY. 



weighin<? 400 lbs. per knot. This is equivalent to 460 grains per 

 foot. The resistance of a wire weighing one grain is found by 

 the tal)le to be O.iMOG; therefore the resistance of a foot of wire 

 weighing 4G0 grains will be ^-^^^-^ ; but the resistance of one knot 

 will be 6,087 times that of one loot ; therefore the resistance re- 

 quired will be suL&l^^o^2Jj^ — 2.79 ohms. If the diameter of the 

 wire be given, instead of its weight i)er knot, the constant is 

 taken fn^ni the second column. Thus the resistance at o2° Fah. 

 of a knot of ])ure hard-drawn copper wire 0.1 inch in diamc-ter 

 would be ^^^Jq''-(^q^^z=:G.05. The resistance of wires is materially 

 altered by annealing them, and a rise in temperature increases 

 the resistance of all metals. Dr. Matthiessen found that for all 

 pure metals the increase of resistance between 32° and 212° Fah. 

 is sensibly the same. The resistance of alloys is much greater 

 than the mean of the metals composing them. They are very 

 useful in the construction of resistance coils. 



The highest value which has probably been fqund for the con- 

 ducting power of pure copper is 60 times that of pure mercury, 

 according to Sabine. Commercial copper may be considered of 

 good quality when its conducting power is over 50. Diflerent 

 samples of copper vary greatly in their specific conductivity, as 

 may be seen by the following table, which gives the result of 

 careful determinations by Dr. Matthiessen, the conducting power 

 of pure copper at 59.9° Fah. being taken as 100. 



Lake Superior, native, not fused, 98.8 at 59.9° 



" " fused (couimcrcial), 92.G at 59.0° 



Burra Curra, 88.7 at 57.2° 



Bc.-it selected, 81.3 at 57.5° 



Bright copper wire, 72.2 at G0.2° 



Tough copper, 71.0 at C3.1° 



Domiduff, 59.3 at 54.8° 



Rio Tinto, 14.2 at 58.6° 



Thus Rio Tinto copper possesses no better conducting power 

 than iron. This shows the great importance of testing the con- 

 ductivity of the wire used in the manufacture of electro-magnets, 

 cables, etc. 



VEGETABLE ELECTROMOTORS. 



The *' Chemical News" contains an article contributed by Ed- 

 win Smith, M.A., giving results of researches in a Held Avhicli, so 

 far as we are aware, has been hitherto untraversed. He says: 

 *' It is well known that a voltaic combination maybe made of 

 two liquids and a metal, if one of the three acts chemically upon 

 one and only one of the other two ; thus, we may employ coi)])er, 

 nitrate of copper, and dilute nitric aciil, or j)latinum, j)otash, and 

 nitric acid. Connect a i)latinum crucible with one terminal of a 

 galvanometer, pour in a little solution of caustic i)otash, place in 

 this the bowl of a tol)acco-pipe iiaving the hole stopped up with 

 wax, ]i()ur into the l)()wl a little nilric acid, dip in the acid a 

 email ^lip pf platinum foil, and connect this with the other ter- 



