94 
PROFESSOR R. THRELFALL AND MR, J. H. D. BREARLEY 
(3) p = 7/5-5 = 1-273, 
(4) p = 7*5/5'5 = 1-364. 
Mean, p = 1*3425 i ‘042. 
[f <f> be the steady deflection corresponding to the throw 0, then (Maxwell, II., 
§ 745) 
Now p — 1*3425. 0 = 7*9, and therefore (f> = 4*53. But (f> is the deflection on 
reversal, hence deflection from the zero for the current employed is 2*26 divisions. 
The current is C = E/It. 
1*43 
1*044 x 10® 
amperes, 
= 2*74 X 10 11 ampere. 
Hence a current of 2*74 X 10~ n ampere produces on reversal a throw of 7*9 micro¬ 
meter divisions. Leaving the damping out of consideration, this is the same as a 
current of 3*5 X 10~ 13 ampere, producing a throw of 1 micrometer division on 
reversal. Taking damping into consideration, this would be slightly increased, since 
the damping is less the smaller the deflection. Now on several occasions (though not 
on this particular one) in times of exceptional magnetic quiet, readings could be taken 
to one-fifth of a micrometer division. If, therefore, we consider that a throw of one- 
fifth of a micrometer division is the least observable, it follows that we could detect 
a current of one-fifth of the above, viz., 7 X 10 -13 ampere, and this with a time of 
swing of only 14"5 sec. In practice we do not think that a current so small as 7 X 10~ 13 
ampere could be detected with certainty, even when the galvanometer is steady, 
because of the small thermo-electric effects that are always exhibited by the contact 
keys, and which would probably give rise to much greater currents than this.* One 
of the most important matters awaiting solution is the construction of a reversing 
key, which shall be free from contact effects. 
If we estimate the current required to produce a steady single deflection of 1 micro¬ 
meter division, we find it is 
c = 
2-74 x 10- 11 2-74 x 10" n 
i ch 
= 1*21 X Hr 11 , 
and the current which would produce the smallest perceptible deflection is one-fifth of 
this, or 2'4 X 10~ 13 ampere. 
On the same day a similar series of experiments were made with currents cor¬ 
responding to 5 X 10 -5 and. 10 X 10 -5 part of the voltage of a Clark cell. 
* Oct., 1893.—Since then we have habitually measured currents of this order. 
