268 Prof. W. Stroud. [June 19, 



By comparing experiments II and III, it would seem that the mag- 

 netic moment of the magnet had altered, probably owing to handling. 

 This alteration does not, however, in any way affect the result, except 

 in so far as the magnet itself expands with rise of temperature. 



Seeing that these results have been obtained with a rough instru- 

 ment made on the premises, and with very inferior mirrors, the 

 method seems very satisfactory. 



For the determination of the strength of a current in absolute 

 measure the writer would suggest placing two coils of wire, each of 

 known geometrical form, the one with its plane in the magnetic 

 meridian on the east side of the instrument, the other similarly on 

 the west side with their axes passing through the centre of the 

 needle. The two coils attached to the instrument would form in fact 

 a Helmholtz standard galvanometer with the addition of the circular 

 magnet with bifilar suspension. Observations of (1) the deflection of 

 the circular magnet on reversal ; (2) the deflection of the needle 

 under the action of the circular magnet ; (3) the deflection of the 

 needle when the circular magnet is removed altogether and the 

 current traverses the coils will give the value of the current in 

 absolute measure correct, it is believed, to one part in a thousand if 

 the geometrical constant of the coils can be determined to that degree 

 of accuracy. 



Now an interesting point arises in connexion with the possible 

 accuracy attainable by this method. The writer believes that with 

 apparatus of the dimensions described there is no difficulty in 

 determining each one of the quantities d, d', I readily to the ToVo part. 

 With a telescope the deflections 0, can certainly be relied upon to 

 that degree of accuracy, at all events if 6, are each more than 5. 

 A little uncertainty arises in connexion with the measurement of r, 

 and this is very important, as H oc r~f. Is the pole to be considered 

 at the middle of the wire of the circular magnet, or nearer the surface 

 of the wire, and if so on which side ? This question cannot be 

 answered with certainty. Reckoning from the middle of the wire in 

 determining the distance between the poles, the maximum error 

 possible in a wire of 0'06 cm. diameter is O'l)3 cm., and this with a 

 radius of 13 cm. gives 1 part in 300 as the extreme error that could 

 be made in H. We may, however, be nearly certain that the pole 

 cannot be more than half the radius of the wire distant from its 

 centre. We may therefore say that about 1 in 500 represents the 

 possible error in H arising from this cause. Clearly, however, it is 

 advisable on all accounts to replace the circular wire of the magnet 

 by a flat steel ribbon bent into the form of a circle. 



It will be noticed that the effect of variation in temperature in 

 altering the value of the constant of the instrument can be allowed 

 for with great accuracy, as the coefficients of expansion of the 



