ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 
679 
It was set up by Mr. Glazebrook for bis experiments by an allied method, and with 
its appurtenances was ready for use at the time that this determination of self-induc¬ 
tion was undertaken. The scale was divided into millimetres, and was placed at a 
distance of 218 centims. from the galvanometer mirror. The instrument was adapted 
for ballistic work, as the vibrations were subject to a logarithmic decrement of only 
about - 0142. 
The electric balance Was provided for by a Resistance box from Messrs. Elliotts. 
The battery current after leaving the reversing key divides itself on entering the box, 
each part traversing 10 ohms. At the ends of these resistances come the galvanometer 
electrodes. The first part of the current now traverses the copper coil, and the second 
part other resistanceSj after which the two parts reunite and pass back to the battery. 
In the use of the “ other resistances,” a special arrangement was adopted which 1 must 
now explain. The resistance of the copper coil being somewhat under 24 ohms, the 
most obvious way to obtain a balance was to add to it a piece of adjustable wire until 
the whole would balance 24 ohms from the box. The objection to this plan is that 
the smallest known disturbance which we can then introduce, i.e., by the addition or 
subtraction of a single unit, is much too great for the purpose. 
The difficulty thus arising is completely met by the use of high resistances, taken 
from a second box 3 in multiple arc with the 24 ohms. 
In order to balance the copper coil aiid its leading wires at the actual temperature 
(about 14°), 753 ohms were required in multiple arc with the 24. To calculate the 
resultant resistance we have 
IT4+T¥3 =‘041666667+-001328021 = •042994688= W2 i 5 -8 ir 9, 
so that the Resistance of the copper coil in terms of the units of the box is 2 3-25 869. 
A suitable deflection 9 was obtained by the substitution of 853 for 753 in the auxiliary 
box. In this case 
Y4+8F3 = '041666667 +•OOIT72333 = -042839000= W ^32 T ; 
so that the additional resistance was 
SQ— ‘08453 unit. 
It may be remarked that if the copper coil had been abolit 1° warmer, its resistance 
Would have been greater by -g+yth part, and the balance would have required 853 
instead of 753 in multiple arc with the 24. 
On account of the progressive changes already mentioned, it was advisable to 
alternate the observations of cc and 6 as rapidly as possible, and to occupy no more 
time than was really necessary in taking the readings. A good deal of time may be 
saved by working the key suitably, and by opening and closing the galvanometer 
branch (at a mercury cup provided for the purpose) so as to avoid producing un¬ 
necessary swings, and to stop those due to induction when done with; but it is 
MDCCCLXXXII. 4 S 
