200 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[May, 



It was Amptrc who first succeeded in effecting the rotation of a magnet 

 round its own ;ixis. He effected this by placing a magnet without support in 

 a vessel of mercury, hut kept in a vertical position by a weight of platinum, 

 attached to its lower end. The object in this experiment was to make 

 the electrical current pass through one half of the magnet, and then, having 

 diverted or broken it from its course, to make it pass away in such a direction 

 as not to affect the other lialf. The reason assigned for this is that supposing 

 a positive current is made to descend a magnet placed vertically, with its 

 north pole uppermost, it would tend to urge that pole round from right to 

 left, but it would have the reverse influence on the south pole, for it would 

 urge it round from left to right. Taking another supposition, that there are 

 two electrical currents, corresponding to the vitrious and resinous electrici- 

 ties, still the tendencies would bo the same. In the experiment just alluded 

 to of Ampere, the electric current after passing through the upper portion of 

 the magnet, then goes into the mercury, through which it is diffused, and in 

 no sensible respect affects the lower portion, nor interferes with the rotation 

 produced by its means on the upper pole. A better mode is, however, now- 

 employed for effecting this object by carrying off the current in a different 

 channel, which is effected by means of an apparatus constructed by Mr. Wat- 

 kins. A flat bar magnet is supported in a vertical position by an upright 

 metal wire, fixed in the base of the apparatus, and having a hole in the cen- 

 tre, containing an agate cup, to receive the lower pointed end of the magnet; 

 its upper end turns in another hole, made in a vertical screw with the milled 

 head to turn it by. This is passed through a screwhole made in an arched 

 piece of wire screwed to the upper part of the platform or base. Around the 

 vertical wire first described a cistern is placed to hold mercury, and another 

 cistern is provided having a hole in its centre, to allow the magnet to pass 

 through, and revolve within it near the middle of the magnet. The cisterns 

 have metal wires projecting into them, through their sides.and supports, cups 

 of mercury, for the purpose of completing the communication by connecting 

 wires with the voltaic battery. Two small curved and pointed wires are fixed 

 to the magnet, and the ends of the wires dip into the mercury in the cistern. 

 The voltaic circuit having been completed the magnet begins to revolve within 

 Ihe electricity, which it conducts itself, forming indeed a part of the circuit. 

 In the.se experiments, the rapidity of the rotations of the magnet depends upon 

 the delicacy of the point of suspension, as much as on the strength of the 

 magnet itself, or the power of the voltaic battery combined. To act upon 

 a large magnet, a cup, to contain mercury, must be provided and fixed to the 

 vertical screw, by means of which and another battery an electric current can 

 be passed from Ihe top of the magnet to its equator, and an opposite current 

 from its lower end to the equator, so that an additional force is obtained. 



The vibratory tendency of electrified « ire is a step beyond this. Mr. Marsh 

 employs a slender wire, suspended from a loop and capable of free motion. 

 Its lower end is amalgamated, and dips into a cistern of mercury. A cup at the 

 top of the wire, and another below it are also filled with mercury, and through 

 them the electric current is passed down the loose wire, but no motion is 

 perceptible until a horse-shoe magnet is placed in a horizontal position on the 

 platform, with its poles enclosing the wire, when the wire is instantly moved 

 backwards and forwards, according to the position of Ihe poles and the di- 

 rection nf the current. The wire thus thrown out of the mercury the circuit 

 is broken, and the effect ceases until the wire falls back by its own weighty 

 when the operation is resumed, and soon produces a succession of vibrations. 

 By employing a spur wheel, with a similar apparatus, this motion can be 

 converted into one of rotation, though after all the division of the wheel into 

 rays is not absolutely necessary, for a circular disc of metal will do quite as 

 well. This is a very interesting and a very pretty experiment, when tried on 

 a considerable scale, as the wheel revolves with immense velocity, and streams 

 of coloured sparks burst from it. 



The rotation of coils, wires, helices, &c., is well illustrated by an appara- 

 tus of Messrs. Knight, of Foster Lane. In Ihe usual plans, mercury is em- 

 ployed, but Messrs. Knight dispense with it, and produce a useful modifica- 

 tion of Ritchie's rotating magnet. 

 "In Fig. 4 a horse-shoe magnet is represented supported on a Iripod stand 

 .^'K- ^- withlevellingscrews. .:/ .^ is the mag- 



net ; B the tripod stand, C C two cir- 

 cular wooden cisterns for holding mer- 

 cury and capable of being adjusted at 

 any reqnjrej height by binding screws, 

 E E are two light w ire frames, F F 

 two helices, // a Ritchie's rotating 

 magnet ; on the tops of the wire frames 

 and helices are small cups to contain 

 a drop of mercury, G is a piece of 

 brass wire bent twice at right angles 

 and terminated at each end by a fine 

 point to dip into the globules of mer- 

 cury : it can be raised or depresst-d 

 without disturbing Ihe general arrange- 

 ment of Ihe apparatus as a simple inspection of the figure will show. 

 " When the rotating magnet is set in aclion in this apparatus a loud hum 



ming noise and sometimes a loud musical sound is excited by the rapid vibra- 

 tory motion assumed by the fixed magnet during the rapid revolution of the 

 electro-magnet. This musical sound is best observed when the levelling 

 screws of the tripod are placed on a mahogany table in the middle of a large 

 room. For tlie electro-magnet i/^ a simple coil of wire maybe substitued, 

 the rotation of which will be exceedingly rapid, its faces becoming alternately 

 attracted and repelled by Ihe poles of the magnet." 

 The power of the electro-magnet is a great consideration, and in order to 



Fig. 5. 



produce the greatest effect of electro-magnetic 

 induction onsi.ftiron, thecurrentmust be made 

 to encircle it by passing through a considerable 

 length of insulated copper wire wound round 

 the iron. A great length of wire is, however, 

 found to weaken the effect of the current, and 

 it is considered better that the total length of 

 the wire intended to be used should be cut into 

 several portions, each of which, covered with 

 silk or cotton thread to prevent lateral commu- 

 nication, is to be coiled separately on the wire. 

 Tile ends of all Ihe wires, Mr. Noad directs, 

 must then be collected into two separate parcels 

 and made to communicate with the same voltaic 

 battery, taking, however, precautions that the 

 current shall pass along each wire in Ihe same 

 direction. The accompanying engraving shews 

 a simple arrangement of the electro-magnet, mounted on a wooden stand with 

 a small scale pan attached to Ihe bit or keeper of Ihe magnet. So intense is 

 the magnetic power thus induced on the iron that weights of upwards of half 

 a Ion can be sustained. Mr. Noad's large magnet, weighing one hundred- 

 weight, will sustain, when excited by an energetic compound battery, from 10 

 to 14 cwt. ; but Mr. Richard Robert's magnet, weighing 35 lbs., with an ar- 

 mature 23 lbs. in weight, when excited by a battery of eight pair of Sturgeon's 

 cast iron jars, is reported to have sustained a weight of 2950 lbs., or upwards 

 of 26 cwt. Mr. Noad's magnet supported a weight of 14 to I, but Mr. Ro- 

 bert's of 84 to I without the armature, or 50 to 1 reckoning the gross weight. 

 We should like to sec such a magnet under the oper.alion of Armstrong's 

 hydro-electric battery at Ihe Polytechnic Institution. At any rate there are 

 indications the sustaining weight is very great. Mr. Radford's magnet, 

 weighing 18J lbs. and with an armature of 14}lbs., e.xcited by a battery of 

 twelve of Sturgeon's cast iron jars, sustained a weight of 2500 lbs., or 22 cwt. 

 The results are as follows :— 



Weight of \V,;ight of 

 Magnet. Armature. 



Noad's magnet 

 Roberts's . . 

 Radford's , . 

 Joule's . . . 



lb. 

 35 



m 



23 

 Hi 



Total 

 Weight. 



111. 



112 

 58 

 32* 



m 



'' Weight 

 sustained, 



lb. 

 1568 

 2950 

 2500 

 2710 



Proportion Proportion 

 to Net to Gross 



Weight of Weight of 

 Magnet. Magnet. 



84 

 137 



lb. 

 14 

 50 

 76 

 236 



The intensity of a magnet of the weight of Mr. Noad's, constructed on the 

 same principle as Mr. Radford's, and excited by an adequate power, would 

 sustain at least twelve tons weight, if Ihe power do not increase, indeed, in a 

 much greater proportion. It is singular, however, that in these experiments 

 the power should be inversely as the weight. Mr. Noad thus describes Mr. 

 Joule's magnet. 



■■The third electro-magnet alluded to, is that of Mr. J. P Joule, and is 



Fig. 6. 



affixed to the lower rin; 

 Fig. 7. 



be very conjiderably augmented." 



shown in Fig 6. B iJ, are two rings of brass, each 

 12 inches in exterior diameter, two inches in 

 breadth and one inch in thickness ; to each of these 

 pieces of iron are affixed, by means of the bolt 

 headed screws, s s,_&ic. : 24 of these are groovcdi 

 and fastened to the upper ring ; 24 are plain and 

 A bundle, IV IV, consisting of sixteen copper wires, 

 each of which was sixteen feet long, and one-twentieth 

 of an inch thick),coveredwithadoublefold of thick col- 

 ton tape, was bent in a zig-zag direction about the 

 grooved pieces. Fig. 7 represents the method adopted 

 for giving, the electro-magnetic ring a firm and equable 

 suspension : a, a, arc hoops of wrought iron, to each of 

 w hich four bars of Ihe same metal are riveted and welded 

 together at Ihe other end into a very strong hook. The 

 hoops are bound down to the brass rings by means of 

 copper wires. The weight of the pieces of grooved iron 

 was 7-025 lbs., and that of the plain pieces 4-55 lbs. ; and 

 when excited by 16 pairs of the cast iron battery, ar- 

 ranged into a series of four, a w-eight of '2,710 lbs. was 

 suspended from the armature, without separating it from 

 the electro-maguet ; and Mr. Joule -thinks, that by the 

 use of some precautions, which have occurred to him 

 since making his first experiments, the actual power will 



