DIMENSIONS OF RINGS AND RODS OF IRON AND OTHKR METALS. 207 



spring C slides from A to B. While the spring is so moving the current from the 

 battery E is being rapidly reversed by the commutator D : thus, the coil upou 

 the ring is traversed by a series of alternating currents of decreasing strength, the 

 effect of which is to completely demagnetise the ring. In practice the slide and the 

 commutator were operated together by simply turning a handle.* 



Fig. l. 



Passing over the earlier and altogether unsuccessful attempts in which the insulated 

 wire was wound directly upon the iron, I give below particulars of three rings which 

 yielded results of value, and which will be distinguished as No. 0, No. 1, and No. 2. 

 The first was of use only for comparatively weak currents : the second and third 

 could, with a little care, be employed in connection with 30 GROVE'S cells. 



Ring No. 0. This was made of soft iron rod of a superior quality, commercially 

 known as " bright rod." The joint was carefully welded and the ring was turned in 

 the lathe to a circular section. It was then covered with four layers of flannel, cut 

 into strips and wound with three coils of insulated wire. 



External diameter of ring 6'50 cm. 



Mean diameter 5*85 cm. 



Diameter of section 0'515 cm. 



Diameter of copper wire . 0*07 cm. 



Total number of turns 450 



Fieldt due to coil 30 X C nearly. 



C being the current expressed in amperes. 



Two brass rods, 6 cm. and 8 cm. in length, were attached by clamps to the exterior 

 of the coils, forming prolongations of a diameter of the ring. These rods served to 

 communicate the variations in the diameter of the ring to the measuring instrument, 

 which will be described further on. 



* The iron was, of course, demagnetised without being removed from the instrument, the slide 

 apparatus being connected when required by means of a switch. 



t Tho field at a distance r from the axis of the ring = 2nt'/r, where n is the number of turns of wire 

 and i the current in C.G.S. units. 



