218 



Prof. D. E. Hughes. 



[May 19, 



copper helices. 50° equalled a copper wire 1 millim. diameter, formed 

 into a helix of 1 centim. diameter, whose spiral turns were separated 

 1 centim. apart. 



In order to obtain a perfect zero, and wide readings, with small 

 angular movement of the coil, it is necessary that the return wire 

 should be of copper, 2 millims. diameter, offering comparatively little 

 resistance, and that it should be perfectly parallel with the steel or iron 

 wire. In order that it may react upon the exterior of the coil, it is 

 fastened to the board, so that it is near (1 centim.) the exterior of the 

 coil, and parallel to the iron wire, at a distance of 4 centims. If we 

 consider this return wire alone, we find, as in the sonometer, that if 

 the wire is perpendicular to the exterior wires of the coil, we have a 

 zero or silence, bat moved through any degree, we have a current 

 proportionable to that degree ; by this means, we have an independent 

 constant acting on the coil, constantly aiding the coil in finding its 

 true zero, and allowing of very wide readings, with a comparatively 

 small angular movement of the coil. 



The rheotome is joined to a battery of two bichromate cells, and by 

 means of a reversing switch, an intermittent current of either direc- 

 tion can be sent through the wire. The telephone is joined direct and 

 alone to the coil ; thus no currents react upon the coil when perpen- 

 dicular to the iron, and its return wire, if not of a spiral nature. 



Placing an iron wire 05 diameter, and passing a current through it, 

 I found a change had taken place similar to those indicated in my 

 paper of March 17th ; but it was so difficult to keep the wire free 

 from magnetism and slight molecular strains, that I preferred and 

 used only in the following experiments tempered steel wire (knitting 

 needles I found most useful). All the effects are greatly augmented 

 by the use of iron wire, but its molecular elasticity is so great that 

 we cannot preserve the same zero of reading for a few seconds together, 

 whilst with steel, 0'5 millim. diameter, the effects remained constant 

 until we removed the cause. 



I have not as yet been able to obtain a steel wire entirely free from 

 magnetism, and as magnetism in steel has a remarkable power over 

 the direction of the\spiral currents, I will first consider those in which 

 I found only traces. On passing the intermittent current through 

 these, the sounds were excessively feeble for either polarity of current, 

 but, at each reversal, a single loud click could be heard, showing the 

 instant reversal of the molecular polarity. The degree of coil indi- 

 cating the twist or spirality of the current was 5° on each side of its 

 true zero. The wire was now carefully magnetised, giving 10° on each 

 side for different currents. The positive entering at north pole indica- 

 ting 10° right-handed spiral, the negative entering the same pole, a left- 

 handed spiral, we here see in another form, a fact well known and de- 

 monstrated by De la Rive by a different method, that an electric current 



