54 Prof; D. E. Hughes on an Induction-balance 



caused by simply rubbing the shilling between the fingers, 

 or the difference of temperature by simply breathing near the 

 coils; and in order to reduce this sensibility within reasonable 

 limits, I have only used in the following experiments 100 

 metres of copper wire to each coil, and 3 cells of battery. 



II. The comparative disturbing value of disks of different 

 metals, all of the same size and form of an English shilling, 

 and measured in millimetre-degrees, by the sonometer, is the 

 following : — 



Silver (chemically pure) 125 



Gold ( „ „ 117 



Silver (coin) 115 



Aluminium 112 



Copper 100 



Zinc 80 



Bronze 76 



Tin 74 



Iron (ordinary) 52 



German silver 50 



Iron (chemically pure) 45 



Copper (antimony alloy) 40 



Lead 38 



Antimony 35 



Mercury 30 



Sulphur (iron alloy) 20 



Bismuth 10 



Zinc (antimony alloy) 6 



Spongy gold (pure) 3 



Carbon (gas) 2 



III. It will be seen from the above that the instrument 

 gives very different values for different metals or alloys ; con- 

 sequently "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 dif- 

 ference by clear loud tones on the telephone. If I place two 

 gold sovereigns of equal w r eight and value, one in each coil, 

 there is complete silence, indicating identity or equality be- 

 tween 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 test any 

 alloy, giving instantly its electrical value. The exceeding 

 sensitiveness of this electrical test I shall demonstrate by ex- 

 periment, now. 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 ui- 



