49 
disposal for absolute measurement and suitable either for practical appli- 
cations or for the most refined laboratory work. Tor the production of 
these we are indebted to a host of inventors. Prominent among them may 
be mentioned Lord Kelvin, Lord Rayleigh, Ayrton and Perry, Mather, 
Swinburne, Cardew and Weston. 
Magneto-electric and dynamo-electric generators and motors have now 
become so common that we are apt to forget that their introduction on 
an extensive scale has only taken a few years. Faraday’s disc dynamo 
Was, as has already been stated, produced in 1851, and a machine for gen- 
erating electricity was made by Pixii in the following year.  Pixii’s 
machine consisted of a horseshoe permanent magnet, which was rotated 
in such a way that its poles passed alternately in front of the poles of a 
similar electro-magnet. An alternating current was thus induced in the 
circuit which included the coils of the electro-magnet. This machine was 
improved by Clarke, who revolved the coils and put a commutator on the 
axle. Other machines were made or suggested by various physicists, and 
an important observation, which has since been frequently overlooked, 
Was made at this time by Jacobi, who pointed out the importance of 
making the cores of the coil short. Sturgeon in 1835 made a dynamo with 
a shuttle-shaped armature; a similar form has long been identified with 
the name of Siemens. Woolrich made a multipolar magneto machine 
in 1841 for electroplating, and Wheatstone about this time produced his 
small multipolar magneto long used for telegraph purposes. In 1845 
Wheatstone and Cooke patented the use of electro-magnets in place of the 
permanent magnets, and Brett suggested in 1848 that the current from the 
machine might be made to pass round a coil surrounding the. magnet and 
thus increase its strength. <A similar suggestion was independently made 
in 1851 by Sinsteden. In 1849 Pulvermacher proposed the use of thin 
laminae of iron for the cores of the magnet, a proposition which has since, 
but probably for a different reason, been almost universally adopted. 
Sinsteden used iron wire cores and made a number of experiments on 
the effect of varying the pole face. About this time another class of 
machines was proposed by Ritchie, Page and Dujardin. In these ma- 
chines both the magnets and the coils were to be stationary, but the mag- 
netism was to be varied by revolving soft iron pieces in front of the poles. 
Modern representatives of these machines are to be found in the dynamos 
of Kingdon, Stanley and others. Ail the machines up to this time had 
4—ScIENCE. 
