458 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Description of Apparatus. 



The shell is in the form of a hollow sphere of cast iron, in the casting 

 of which every precaution was taken to have the iron free from blow 

 holes and of uniform quality. It is divided into two hemispheres by a 

 plane passing through a great circle. A narrow rim on one hemisphere 

 accurately fitting a corresponding groove in the other, insures the perfectly 

 concentric position of the two parts at all times. A radial hole is pro- 

 vided for the admission of lead wires to the heating coil and thermo- 

 electric junctions, and another for the shaft which drives the fans in the 

 interior of the sphere. 



The sphere is coated by electrolysis with copper to a uniform thickness 

 of about 2 mm., it having been shown by Professor Hall 1 that this thick- 

 ness would be sufficient to prevent the lead wires from conducting away 

 sufficient heat to change the temperature in the immediate vicinity of the 

 copper-iron junction. 



These lead wires, eight in number, four inside and four outside, are 

 electroplated to the copper. The two from the inside of each hemisphere 

 are brought through a wood fibre plug split in the same plane as the 

 sphere. These are then united with the outside wires, and the whole 

 wrapped with a thin silk ribbon, thus forming a small, well protected 

 cable. The ends of the wires are brought to a block of fibre 3" X 1" X f " 

 provided with eight small holes arranged in two rows h" apart. The 

 wires from the outside are put in one row, and the corresponding ones 

 from the inside in the other. The holes are then filled with mercury and 

 a cover of fibre with a similar set of holes screwed on the block to hold 

 the wires in place. 



The leads from the galvanometer are brought to this block and so 

 arranged that any pair of junctions can be connected and reversed without 

 approaching the block or galvanometer. The DArsonval galvanometer 

 used has a resistance of about 600 ohms and gives a deflection of about 

 160.0 mm., readable to tenths of a mm., with a potential difference of 

 .0001 volt, the scale distance being 3.3 meters. It is mounted on a 

 Julius device and carefully protected from mechanical disturbances. The 

 galvanometer and leads to the junctions are packed in hair felt to insure 

 a constant temperature. 



The temperature of one of the junctions must be known. This is ac- 

 complished by means of a thermo-electric couple consisting of an iron and 



1 These Proceedings, 31, 1895. 



