274 SCIENTIFIC RECREATIONS. 



poles upwards, revolves a magnet. The commutator is fixed on the same 

 axle and revolves with the permanent magnet. Saxton, and subsequently 

 Clarke, made the obvious improvement of making the magnet less cumbrous- 

 and fixed, and causing the bobbins of the electro-magnet to revolve before or 

 rather beside its poles ; the commutator was fixed at the end of the axle on 

 which the revolving bobbins (or armature) are fixed. Niaudat formed a. 

 compound Clarke machine, by setting two horse-shoe magnets a short 

 distance apart. The armature revolves between them, and consists of 

 twelve coils set between two plates; the coils are set alternately and con- 

 nected, i.e., the poles of the electro-magnets are set beside one another, 

 N. to S., S. to N., and so on, so that the N. pole receding produces a current ;, 

 but the N. pole receding makes the S. pole approach, and produces another 

 current, A B ; in fact, a continuation of the same, for the approach of a. 

 N. pole naturally produces the same current as the recession of a S. pole ;, 

 then as the S. pole in turn recedes it produces an inverse current, B A, which, 

 is in turn kept up by the approach of the next N. pole, and so on. Each coil 

 is attached to a radiating metal bar, which conveys the current to be redirected 

 to the commutator, which is affixed to the axle of the revolving armature as- 

 in Clarke's machine. In 1854 Siemens completed his machine, the chief 

 peculiarity of which was its cylindrical bobbin ; the core is grooved deeply,, 

 parallel with its axis, and the wire is wound on cylindrically and covered 

 with plates of brass ; one end of the coil is fixed to the metal axis, the 

 other to an insulated ferule at the end of the axis, where is also situate the 

 commutator. This armature revolves between the poles by which it is 

 closely embraced. One of the most celebrated of the magneto-electrical 

 machines is that known as the "Alliance," invented by Nollet,and perfected by 

 Van Malderen. It is composed of four or six bronze discs, revolving on an* 

 axle, round the external circumference of each of which are set sixteen 

 bobbins. This rotating compound armature revolves between four to six 

 sets of horse-shoe magnets, which, being fixed radially to the centre, present 

 in each set sixteen poles to the sixteen bobbins. It will be readily under- 

 stood that this immense quantity of poles and bobbins produces a highly 

 concentrated current, the ends of which proceed from the axle and an* 

 insulated ferule at its extremity. 



In 1869 Mr. Holmes perfected his machine, which differs from all pre- 

 vious ones (except Pixii's), in that the electro-magnets revolve in front of 

 the coils instead of vice versd; and besides magnetising his electro-magnets 

 with part of the self-produced electricity, his bobbins are so disposed as to 

 be able to keep several independent lights going at once. The Wylde 

 machine consists, as it were, of two Siemens machines, one on the top of the 

 other, the lower and larger of which is worked by an electro-magnet, which 

 is magnetised by the action of the upper or smaller one, consisting in the 

 ordinary way of a permanent magnet apparatus, which is termed " the excit- 

 ing machine." The longitudinal bobbin revolved between these permanent 

 poles produces alternating currents, which are commutated (or redirected), and 



