60 



S : >fr YjiezriiTillv p~? r" 

 Gold 



Silver (coin) 



Almrn'TiinTn 



Iron (ordinary) 



P: ::. D. E. E-igLrS. 



... 125 



... 117 



... 115 



. .. 112 



. .. 100 

 80 



... 76 



... n 



52 

 50 



[May 15, 



Iron (chemically pare) 45 



Copper (antimony alloy) 40 



Lead 38 



Aurini'iiv 35 



Mercury.. 30 



Sulphur (iron alloy) 20 



Bisr^irl 10 



Zin: s,:: rln : nv illovi 8 



>?:--*y p^e 3 



Carbon g^s 2 



These numbers do not agree entirely with any lists of electrical 

 conductivity I have yet met with; the nnmbers are, however, in- 

 variably given by the sonometer, and the divergence may be dne to 

 some peculiarity of structure of the metals when formed into disks. 

 Future investigations with this instrument will, no doubt, give more 

 correct values than I have been able to obtain with my limited means 

 of research. 



in. It will be seen from the above, that the instrument gives very 

 different values for different metals or alloys, consequently, we cannot 

 obtain a balance by employing two disks of different metals, and the 

 instrument is so sensitive to any variation in mass or matter, that it 

 instantly detects the difference by clear loud tones on the telephone. 

 If I place two gold sovereigns of equal weight and value, one in each 

 coil, there is complete silence, indicating identity or equality between 

 them ; but if one of them is a false sovereign, or even gold of a 

 different alloy, the fact is instantly detected by the electrical balance 

 being disturbed. The instrument thus becomes a rapid and perfect 

 coin detector, and can assay any alloy, giving instantly its electrical 

 value. The exceeding sensitiveness of this electrical test I shall 

 demonstrate by experiment this evening. Again, as regards coins, it 

 resolves an almost magical problem. Thus, if a person puts one or 

 several coins into one pair of coils, the amount or nominal value being 



different coins successively, as I should weights in a scale, and when 

 perfect balance is announced by the silence, the amount in one box 

 will not only be the same nominal value but of the same kind of coin. 



IV. We find by direct experiment with this instrument, that the 

 preceding results are due to electric currents, induced by the primary 

 coil, and that it is by the reaction of these that the balance is 

 destroyed, for, if we take an insulated spiral disk or helix of copper 

 wire, with its terminal wires open, there is no disturbance of the 

 balance whatever, notwithstanding that we have introduced a com- 

 paratively large amount of copper wire ; bnt on closing the circuit, 

 the balance is at once very powerfully disturbed. 



If the spiral is a flat one, resembling a disk of metal and circuit 

 closed, we find that lond tones result when the spiral is placed flat, or 



