372 ELEMENTARY LESSONS ON [CHAP. x. 



to generate currents in place of voltaic batteries. In 

 the early attempts of Pixii (1833), Saxtori, and Clarke, 

 bobbins of insulated wire were fixed to an axis and spun 

 rapidly in front of the poles of strong steel magnets. 

 But, since the currents thus generated were alternately 

 inverse and direct currents, a commutator (which rotated 

 with the coils) was fixed to the axis to turn the successive 

 currents all into the same direction. The little magneto- 

 electric machines, still sold by opticians, are on this 

 principle. Holmes and Van Malderen constructed more 

 powerful machines, the latter getting a nearer approach 

 to a continuous current by combining around one axis 

 sixty-four separate coils rotating between the poles of 

 forty powerful magnets. 



In 1857 Siemens devised an improved armature, in 

 which the coils of wire were wound lengthways along 

 a spindle of peculiar form, thereby gaining the advantage 

 of being able to cut a greater number of lines -of -force 

 when rotated in the powerful " field " between the poles 

 of a series of adjacent steel magnets. The next im- 

 provement, due to Wilde, was the employment of elec- 

 tromagnets instead of steel magnets for producing the 

 " field " in which the armature revolved ; these electro- 

 magnets being excited by currents furnished by a small 

 auxiliary magneto-electric machine, also kept in rotation. 



4O8. Dynamo-electric Machines. In 1867 the 

 suggestion was made simultaneously, but independently, 

 by Siemens and by Wheatstone, that a coil rotating 

 between the poles of an electromagnet might from the 

 feeble residual magnetism induce a small current, which, 

 when transmitted through the coils of the electromagnet, 

 might exalt its magnetism, and so prepare it to induce 

 still stronger currents. Magneto-electric machines con- 

 structed on this principle, the coils of their field-magnets 

 being placed in circuit with the coils of the rotating 

 armature, so as to be traversed by the whole or by a 

 portion of the induced currents, are known as dynamo- 



