50 Scientific Proceedings, Royal Dublin Society. 



junctions are usually formed at the binding-screws between brass and 

 copper or between two different samples of copper. Strained places even 

 in the copper leads ma}' also act as thermo-junctions. Another source of 

 trouble is strains in the galvanometer suspension, whicli lead to continual 

 changes in the position of the zero on the scale. The slowness of the 

 galvanometer needle to take up its final ])osition may also be mentioned as 

 introducing uncertainty in deciding on the true magnitude of the deflection. 



In view of these sources of error, it is evidently of great importance to 

 have as few connections in the circuit as possible, and, where such connections 

 are unavoidable, to secure that they are balanced by similar connections kept 

 at the same temperature. 



In order to eliminate one usual set of connections from the circuit, i.e. 

 that between the thermo-couple and the leads, in the apparatus which 

 Mr. Atkins and I used, it was arranged to utilize the ends of the copper 

 leads themselves as one pair of elements in the junctions. These leads, which 

 had a diameter of 0'17 mm., extended right from the junctions to' the 

 reversing key (to be described later on). The otlier pair of elements of the 

 couple were formed of the ends of a continuous iron, nickel, german silver, 

 or ' eureka ' wire. 



In most of the work on whicli we were engaged eureka-copper junctions 

 were the most extensively used. The eureka alloy has a high thermo-electric 

 value when forming a junction with copper, and so is capable of giving a 

 large deflection for a small temperature difference. Its comparatively great 

 resistance enables one to adjust very conveniently the sensitiveness tlius 

 produced by increasing or diminishing the length of the eureka in the couple. 

 Its low coefficient of variation of resistance with temperature secures that this 

 convenient resistance introduces practically no error ; and when, as in the 

 apparatus to be described, it is enclosed in the freezing-cliamber, the error is 

 so small that it may be disregarded. 



The specific resistance of eureka is given as 47400 O.Q-.S. units ; its 

 variation per 1° 0. as 0'0048 per cent. 



The construction of the eureka copper thermo-couple is simplicity itself. 

 To each end of the silk-covered piece of eureka wire about 1 ra. long a 

 convenient length of the copper lead is soldered. The eureka wire I made 

 use of liad a diameter of 0'19 mm., and a resistance of about 16*5 ohms 

 per metre. The soldered junctions between the eureka and copper may 

 be neatly made by stripping a few millimetres of the ends of each from their 

 silk coverings and dipping the bared tips into a solution of resin in spirit. 

 After this treatment, if the ends in contact with one anotlier are immersed 

 in a tiny drop of molten solder, a very compact and good junction is made. 



