154 ANNUAL OF SCIENTIFIC DISCOVERY. 



much practical importance, but of high scientific interest. If chemical com- 

 position is to be looked to for the explanation, very slight deviations from 

 perfect purity must be sufficient to produce great effects on the electric con- 

 ductivity of copper, the following being the results of an assay made on one 

 of the specimens of copper wire of low conducting power : 



Copper. ' 99-75 



Lead -21 



Iron -03 



Tin or Antimony -01 



100-00 



The entire stock of wire from which the samples experimented on were 

 taken, has been supplied by the different manufactories as remarkably pure ; 

 and being found satisfactory in mechanical qualities, had never been sus- 

 pected to present any want of uniformity as to value for telegraphic purposes, 

 until Professor Thomson discovered the difference in conductivity referred 

 to in his paper. Experiments show that the greatest degree of brittleness 

 produced by tension does not alter the conductivity of the metal by as much 

 as one half per cent. Experiments also showed that no sensible effect was 

 produced on the conductivity of copper by hammering it flat. The author 

 has not yet been able to compare very carefully the resistances of single 

 wires with those of strands spun from the same stock, but it is certain that 

 any deficiency which the strand may present when accurately compared with 

 solid wire, is nothing in comparison with the differences presented by differ- 

 ent samples chosen at random from various stocks of solid wire and strand 

 in the process of preparation for telegraphic purposes. 



ON THE ELECTKO-DYNAMIC INDUCTION MACHINE. 



At the Dublin meeting of the British Association, Professor Callan, of 

 Maynooth College, presented the result of a long series of experiments on 

 the electro-dynamic induction machine. The first of these results is a means 

 of getting a shock directly from the armature of a magnet at the moment of 

 its demagnetization, by using, not a solid piece of iron, but a coil of very 

 fine insulated iron for the armature of an electro-magnet, between the poles 

 of which the coil would fit. When the helix of the magnet is connected 

 with a battery, the armature is magnetized on account of its proximity to 

 the magnetized iron ; and when the battery connection is broken, if the ends 

 of the insulated iron wire be held in the hands, a shock will be felt. The 

 second result is the discovery of the fact, that if iron wires be put into a coil 

 of covered copper wire, the ends of which are connected with a battery, and 

 if another coil be connected with the same battery, the quantity of electricity 

 which will flow through the latter will be greater when the first coil is filled 

 with iron wires than when they are removed. The third result is, a core for 

 the primary coil, which consists of a coil of insulated iron wire, and which 

 has five advantages over all the cores in common use. First, there is no 

 complete circuit for any electrical current excited in any section of the core, 

 because all the spirals of the coil are insulated from each other, and no spiral 

 returns to itself. In the common cores, even when the wires are covered 



