[ 542 ] 



LIV. A New Magnet-testing Instrument. By A. Bursill, 

 B.Sc. (/%.), A.M.I. E.E.I and H. Bedsox, A.M.I.E.E.* 



[Plate VIL] 



DURING the course of a research on magnetos carried 

 out at Padd'mgtoii Technical Institute, it became 

 necessary to develop an accurate and rapid method of 

 measuring the strength or the permanent magnets employed. 

 As the result of considerable experimental work in this 

 direction, an instrument was designed and constructed 

 which was found in practice to give consistent and accurate 

 measurements of the magnetic flux of the magnet under test, 

 the conditions very closely approximating to those which 

 obtain in the magneto itself. "With the addition of suitable 

 coils the magnetic quality of the steel of which the magnet 

 was composed could be tested also. 



Most manufacturers of magnetos or magnet makers have 

 som^ kind ot magnet-te.-ting apparatus, but as a rule a rough 

 and ready method of comparison is deemed satisfactory, and 

 it is doubtful if any two such instruments \* ould be found to 

 give similar results with the same magnet as a purely arbitrary 

 scale is commonly employed. 



On the other hand, it is beyond the range of practical 

 testing to apply the usual laboratory methods to the test of 

 magneto magnets, as this involves laborious experimental 

 work unsuited to rapid repetition, and moreover the form of 

 specimen being fixed by the requirements of the magnet 

 renders the determination of the theoretical quantities involved 

 in the laboratory method of doubtful value. 



In designing- the instrument to be described here the 

 object has been frankly practical, while the quantities mea- 

 sured are expressed directly in scientific units, and the 

 results agree to a satisfactory degree of accuracy with those 

 found by lengthy experimental determination. 



Measurement of Flux. 



The instrument consists mainly of a small hand-driven 

 direct-current generator, wound to generate an unvarying- 

 voltage of about 100 when the magnetic flux is due to an 

 ordinary magneto magnet and the speed normal. The 

 armature is driven from the handle by suitable gearing, and 

 has an upper limit of speed owing to the drive taking place 

 through a clutch which operates by centrifugal force at the 



* Communicated bv Dr. J. H. Vincent. 



