()// a Sew Magnet-testing Instrument. 543 



-critical speed. This generator has been developed to a high 

 degree of precision by Messrs. Evershed and Vignoles for 

 the purpose of their insulation-testing instrument, and pro- 

 vided that the handle is rotated above the operating speed of 

 the clutch the armature is driven at a fixed speed. 



The generator is provided with soft iron pole-pieces having 

 the distance between the outer parallel faces such that the 

 magnets slip on in the same way as they do in the case of a 

 magneto. The diameter of the armature, length of air-gap, 

 and distance between the outer faces of the pole-pieces are 

 made to standard magneto specification, and are accurate to 

 i he standard magneto limits. Should it be desired to make 

 •the same instrument suitable for testing more than one size 

 of magnet, this can be done by providing suitable parallel 

 iron distance pieces to make a good fit between the fixed 

 pole-pieces and the ground surfaces on the magnet limbs. 



In circuit with the armature are connected a suitable 

 delicate current-measuring instrument (a small moving coil 

 mill i voltmeter is actually used), and a resistance-coil of high 

 resistance which is adjusted during the calibration of the 

 instrument. This instrument, comprising the complete 

 flux-measuring set, is portable, the weight when fully 

 assembled in a robust polished mahogany case provided with 

 a leather handle being 111b. PI. VII. fig. 1 is a view of the 

 instrument with a magnet in position ready for determination 

 of flux. 



Calibration. 



In addition to the usual winding on the armature there is 

 a coil of a known number of turns so wound that it can be 

 made to embrace the whole magnetic flux through the 

 armature. This is a search-coil for the purpose of cali- 

 bration. 



Fig. 2 is a diagram of connexions for calibration purposes. 



To carry out the calibration a permanent magnet is 

 slipped upon the pole-pieces to provide the necessary mag- 

 netic flux, and the search-coil terminals connected to either 

 a ballistic galvanometer G, or a fluxmeter by flexible wires. 

 The armature is now turned through 180°. starting and 

 finishing in such a position that the whole flux is just 

 reversed through the search-coil. By comparing the throw 

 on the ballistic galvanometer with that produced by reversing 

 the current in a standard solenoid S provided with a secondary 

 winding, the total flux through the search-coil and therefore 

 ■through the armature may be calculated readily. 



