444 Messrs. Ayrton and Mather on Galvanometers, 



perty of the new coils allows a magnet to be used whose field 

 is four times as strong as those ordinarily employed in this 

 type of instrument." 



Bather more than a year later, in the i Electrical Engineer ' 

 for December 13th, 1895, further information on this subject 

 is given : — u This phenomenon was first noticed by Mr. Mather 

 in December 1891 ... The coil to be tested was suspended 

 in the earth's field and the period of a complete oscillation 

 was found to be 25*2 seconds. It was then placed in a field 

 of 1500 C.G.S. lines and the period was only 3*5 seconds," 

 corresponding with a controlling force dne to the magnetic 

 action of the coil fifty times as great as that due to the 

 suspension itself ..." parts of the coil became magnetic and 

 it can easily be seen that this secondary effect is magnified in 

 the d'Arsonval type of galvanometer, when, with the object 

 of gaining sensibility, the magnetic field is strengthened . . . 

 in fact the magnetic action of the coil will defeat this attempt.'" 



References are also given in the ' Electrician ' and ' Elec- 

 trical Review ' of the same date to the limitation in sensi- 

 bility produced by the magnetism of the coil ; and lastly, in 

 the ' Electrician ' for January 31st, 1896, Mr. Fisher, in a 

 series of articles on the " Crompton Potentiometer," writes: — 

 '•' whilst up to a certain point the deflexions became greater 

 with increased strength of field, beyond that point the de- 

 flexion gradually fell off as the field was strengthened, this 

 being due, as Messrs. Ayrton and Mather have pointed out, 

 to the presence of iron in the materials used in the construc- 

 tion of the coils. A remedy for the same was found by 

 Messrs. Ayrton and Mather, and it is due to Prof. Ayrton's 

 kindness in disclosing the process adopted that the sensi- 

 bility of the instrument is to a great extent due." 



We may take this opportunity of replying to an article on 

 " Galvanometer Design. Waste Space near the Needle," 

 published in the Phil. Mag. for December 1895, by Prof. S. W. 

 Holman, of the Massachusetts Institute of Technology, as a 

 criticism of one of the facts brought out in the 1890 paper on 

 " Galvanometers" by Dr. Sumpner and ourselves. Jn that 

 paper it was pointed out that to obtain maximum sensibility 

 with an ordinary reflecting-galvanometer no wire should be 

 wound in a certain space close to the needle, or, if wound, it 

 should be connected up the reverse way to the rest of the coil; 

 and it was shown that the approximate shape of the space in 

 question was " an oblate spheroid with a polar axis about 0*72 

 of its equatorial diameter, the latter being, of course, slightly 

 larger than the length of the needle." 



