1886.] On the Self-induction of an Electric Current, 453 



this purpose ; the end of the helix E joins directly with the terminal 

 N ; the wire to be tested X is joined to N and H ; from H to I there 

 is a second supplementary German silver wire allowing ns by means 

 of the contact side M, which is in direct communication with B, to 

 introduce more or less of the German silver wire in the side AB. 

 The resistance of the wire to be tested should always be less than 

 that of the opposite side of the bridge, and we then make up the total 

 resistance of AB by sliding the contact slide M until the resistance of 

 AB equals AD. 



It will be evident that in the whole series of experiments there 

 can be no change in the resistance of either side whenever zero is 

 found, the induction coils or balance having the same proportional 

 current in all the changes of the wires under observation, the battery 

 and telephone circuit present also a continuous absolute relation ; we 

 may, however, when desirable, keep the resistance from H to M a 

 constant, and vary the resistance or the length of the German silver 

 wire GD, we then balance the wire X by an equal resistance on the 

 opposite side of the bridge, but then the battery and telephone 

 circuits no longer possess the invariable relations which are so 

 extremely necessary in experiments of the nature of which I have 

 been making. 



The battery circuit is joined in the usual manner to AC, the current 

 can be interrupted by the rheotome K, or by means of a commutator 

 (not shown in the diagram) close the battery circuit, and transfer the 

 contact maker to the telephone circuit BD ; this allows us to observe 

 the effect of an intermittent current compared with that of a constant 

 or steady flow. M. Gaugain has termed the period of a steady flow of 

 current the stable period, and that in which a rise or fall of the 

 current takes place the variable period; these terms have since been 

 generally adopted by telegraph electricians, and in order to avoid 

 introducing new terms, they will be used in this paper. 



It would require too much space to enter into the details of the 

 construction of the bridge, special care is required in the construction 

 of the balancing induction coils, and in securing perfect electrical 

 contacts in all parts of the bridge ; the induction balance requires 

 calibration, and for this purpose I introduce as the wire X successive 

 lengths of 10 cm. 1 mm. diameter copper wire, thus forming a table 

 of values throughout the range of the induction balance, or by equal 

 increments of 10 cm. of copper wire up to 20 metres. 



Having no unit of self-induction to which my results could be 

 quickly and practically referred, I have adopted as unit the self- 

 induction of a straight copper wire 1 mm. diameter and 1 metre in 

 length ; this gives on my calibrated induction scale 100 degrees, and 

 it is to this standard that all the comparative force of extra currents 

 mentioned in this paper is compared. 



