﻿MS Dr. B. S. Willows on the 



Rayleigh was adopted *. E is a wooden ring of 4 inches 

 diameter, F one of 2 inches ; the smaller can be rotated 

 round a common diameter of the two. Each ring carries three 

 turns of copper wire, 0'33 mm. diameter, joined in series, 

 and the whole arrangement is in series with A. By rotating 

 the coil F, the induction in the arm could he varied within 

 narrow limits. A similar pair of coils is in series with B. 

 By this means the balance point on the wire of the bridge 

 with alternating current could be found readily to 1 mm., 

 generally much nearer, and as the resistance of 1 mm. of the 

 bridge wire was 0*0006 ohm, this corresponds to a minimum 

 accuracy of about 0'02 per cent. 



Various current interrupters were used. In the preliminary 

 experiments a secohmmeter and the galvanometer were tried, 

 but soon abandoned, because thermo-currents were trouble- 

 some, and also because I desired to get a greater range of 

 frequency. Finally a vibrating wire with mercury contact 

 was used in the low-frequency experiments. This broke the 

 primary circuit of a small induction-coil, the secondary of 

 which was joined to the bridge, and it possessed the advantage 

 that it could readily be tuned to unison with the vibration- 

 galvanometer used as detector. For most of the other 

 observations a wheel interrupter was used. This was made 

 by letting into the circumference of a disk of beech-wood, 

 1 foot in diameter, about 120 pieces of brass whose width along 

 the circumference was about y 1 ^ inch, the whole being 

 carefully turned. Pressing on the circumference were two 

 springs which completed the circuit through the brass pieces 

 as the wheel revolved when driven by a motor. A small 

 condenser was connected to the springs to prevent sparking. 



The bridge was balanced first with alternating current by 

 shifting the movable contact and adjusting the coils E, F 

 until the inductances as w r ell as the resistances of the arms 

 were equal ; then with direct current. The coils A and B 

 were then generally interchanged and the observations 

 repeated. Mr. F. G. Bratt helped me considerably by taking 

 readings alternately with me. 



The alloys used were eureka, brass, platinoid, German 

 silver, platinum-iridium, and platinum-silver. The frequency 

 of the current varied between 10 and 980 per second. 



No certain differences could be detected between the 

 resistance of* an alloy to direct and alternating current at 

 temperatures 20° and" 100° C. 



As already mentioned, Fleming and Dewar found the 

 resistance of a pure metal very small at low temperatures, 



* Phil. Ma^ Dec. 1886. 



