60 



Dr. J. HopMnson and Mr. E. Wilson. [Mar. 17, 



The following table gives the leading dimensions of the machine : — 



cm. 



Length of magnet limb 66*04 



Width of magnet limb 11*43 



Breadth of magnet limb 38*10 



Length of yoke 38*10 



Width of yoke 12*06 



Depth of yoke 11'43 



Distance between centres of limbs 23*50 



Bore of fields 21*21 



Depth of pole-piece 20*32 



Thickness of gmi-metal base 10*80 



Width of gap 12*06 



The armature core is built up of soft-iron discs, ~No. 24 B.W.G., 

 which are held between two end plates screwed on the shaft. 



The following table gives the leading dimensions of the armature : — 



cm. 



Diameter of core 18*41 



Diameter of shaft 4*76 



Length of core 38*10 



The core is wound longitudinally according to the Hefner von 

 Alteneck principle with 208 bars made of copper strip, each 9 mm. 

 deep by 1*8 mm. thick. The commutator is formed of fifty-two hard- 

 drawn copper segments insulated with mica, and the connexions to 

 the armature so made, that the plane of commutation in the commu- 

 tator is vertical when no current is passing through the armature. 



Each dynamo is intended for a normal output of 80 amperes 

 140 volts, at 880 revolutions per minute. The resistance of the 

 armature measured between opposite bars of the commutator is 

 0*042 ohm, and of each magnet coil 13*3 ohms. 



In the machine, the armature core has a greater cross-section than 

 the magnet cores, and consequently the magnetising force used 

 therein may be neglected. The yoke has the same section as the 

 magnet cores, and is therefore included therein, as is also the pole- 

 piece. The formula connecting the line integral of the magnetising 

 force and the induction takes the short form 



where 



n is the number of turns round magnet. 



c is the current round magnet in absolute measure. 



4 the distance from iron of armature to rim of magnet. 



* ' Phil. Trans.,' p. 335. 



