GOG Prof. Richardson and Mr. Lownds on tlie Magnetic 



throughout the experiments. This circuit could be connected 

 when desired to the galvanometer terminals by means o£ the 

 rocker Q. 



The galvanometer was a Crompton Midget D'Arsonval^ 

 and was suitable for ballistic or deflective zero work. 



This instrument consists of a circular coil suspended bifilarly 

 between the poles of a permanent magnet. The suspending 

 fibres, which are of bronze, serve to carry the current. The 

 galvanometer was very dead-beat and consequently the throws 

 could be read with considerable ease. 



The secondary circuit was kept closed throughout a series 

 of readings, and the throws were always taken in the same 

 direction. The kind of accuracy obtainable with this instrument 

 can be seen from the following observations. The primary 

 of the standard mutual inductance-coil was connected to the 

 battery, and three successive readings of the kick obtained on 

 reversing the current in the primary w T ere taken for currents 

 of different strengths. 



The following set of readings were obtained : — 



Current in 

 Amperes. 



Tli rows. 



Throw 

 -r current. 



1. 



2. 



3. 



Mean. 



150-7 



1-42 



152 



149 



151 



106:1 



1075 



114-5 



113 



115 



1142 



106-2 



0-782 



83 



83 



82-5 



82-8 



1050 



0-27 



28-5 



29 



28-5 



28-7 



106-3 



It will be seen from this table that the instrument is suit- 

 able for ballistic work. 



The value of the standard mutual inductance-coil was 

 533,626 c.G.s. units. Its value was obtained by Mr. C. Gr. 

 Lamb by comparison with a standard coil kept in the 

 Engineering Laboratory at Cambridge. 



§ III. The Measurement of Temperature. 



The method used for measuring the resistance of the ther- 

 mometer-wire was the same, in all essentials, as that described 



