Form of Dynamo-Magneto-Electric Machine. 73 



whole of the current is obtained in the best condition for heating 

 wires, decomposing water, giving an electric light, and other usual 

 experiments. 



At present a model machine has been constructed on this prin- 

 ciple, the armature of which measures 5 inches long by 2 inches 

 diameter, on which is wound about 50 feet of cotton-coyered copper 

 wire, no. 16, B. W. Gt. The magnet has about 300 feet of covered 

 copper wire, no. 14, B. W. Gr. : the whole instrument, without 

 the driving-gear, weighs 26 lbs. ; with this apparatus 8 inches of 

 platinum wire, -005, can be made red-hot, water is rapidly decom- 

 posed, &c. 



The armature is constructed specially to prevent the accumulation 

 of heat to which every class of dynamo-magneto-electric machine is 

 liable. It is made in two halves, a groove of a zigzag form being 

 cast in each half, so that when the two are screwed together a 

 continuous channel is maintained through the bearings for a cur- 

 rent of cold water to pass during the whole time the machine is at 

 work. 



The advantages suggested by these arrangements are their ex- 

 treme simplicity, the few number of parts, only one armature and 

 one wire being used. 



This principle of the alternate current being utilized is also 

 applicable to machines constructed on the multiple armature prin- 

 ciple; and the economy thereby resulting would prove of great 

 advantage, as the power of the machine could be varied by throwing 

 into the electromagnets either every other current, or every fourth, 

 sixth, or eighth current, according to the strength required in the 

 machine, the whole of the other currents being utilized for electric 

 light or otherwise. 



" On the Determination of Verdet's Constant in Absolute Units. " 

 By J. E. H. Grordon, B.A., Gonville and Caius College, Cambridge*. 



In the year 1845 Faraday discovered that certain media possess 

 the property of rotating the plane of polarization of light passing 

 through them when a magnetic force acts on them. About the 

 year 1853 M. Yerdet found that with the same magnet and medium 

 the rotation is directly proportional to the strength of the magnet 

 — that is, that the ratio between the amount of rotation and the 

 intensity of the magnetic field is constant. 



The object of this investigation is to measure this constant in 

 absolute units for a standard substance. Distilled water was used, 

 and the magnetic force was produced by means of an electric 

 current in a helix, as the magnetism of iron magnets is an unde- 

 termined function of the shape and nature of the iron core. 



The strength of the helix was determined by comparing the 

 magnetic force at a series of seven equidistant points along its 



* The whole of this work has heen done under Prof. Clerk Maxwell's super- 

 intendence. He suggested the method and nearly all the details; and any 

 merit which the investigation may have belongs to him. He is, however, in no 

 way responsible for any errors there may be in the numerical results. 



