26 Prof. J. A. Fleming's Design for a 



form of the standard, and one form which has proved itself to 

 be very satisfactory in use is as follows : — 'The case or shell 

 which contains the coil is in the form of a ring (see fig. 5). 

 This ring consists of a pair of square-sectioned circular troughs 

 provided with flanges which can be screwed together so as to 

 form a square-sectioned, hollow, circular ring. 



From this ring proceed upwards two brass tubes about 

 five or six inches in length. Down these brass tubes pass the 

 copper electrodes or rods, and these rods are insulated from 

 the tubes at the top and bottom by ebonite insulators. The 

 insulator at the bottom of the tube, where it enters the ring, 

 is a simple collar, that at the top has the form of a funnel 

 corrugated on its outer surface. The use of this funnel will 

 be referred to presently. The actual resistance-coil is a 

 length of platinum-silver wire three-fold silk-covered. The 

 silk-covered wire is first baked above 100° C. to dry it com- 

 pletely, and then immersed in melted ozokerit or paraffin. 



The so insulated wire is cut about the proper length and 

 laid double or folded once upon itself and then rolled up on 

 a wooden mandril so as to form a circular coil of diameter 

 suitable to drop into the hollow of the brass ring. The wire 

 being wound double, its coefficient of self-induction is rendered 

 very small. This coil of wire is then wrapped over with 

 white silk and again dipped in melted ozokerit. The ends of 

 the wire are next soldered into nicks in the ends of the copper 

 rods, they having been previously pushed a little way through 

 the brass tubes for the purpose, and afterwards drawn back 

 into proper positions. The coil is then packed into the 

 circular groove, and, after adjusting the resistance to the 

 proper value, the bottom half of the ring is placed over it. 

 A thin washer of indiarubber is inserted between the flanges, 

 and the whole screwed tightly together. The resistance-coil 

 is thus enclosed in a thin ring of metal, and can be placed 

 wholly below the surface of water or ice. In order to test 

 the tightness of the joints, a little test-pipe is provided on the 

 upper surface of the ring. By placing the ring coil below 

 water and blowing into the test-pipe, the good fitting of the 

 joints can be assured. The aperture of this test-pipe is after- 

 wards closed by solder or a screw (see fig. 6). 



Apart from the insulation of the coil itself it will be appa- 

 rent that the insulation is limited by the amount of insulation 

 resistance secured at the ebonite insulators at the top end of 

 the brass tubes. Any leakage from the copper rod over these 

 insulators to the brass tube destroys to that extent the insula- 

 tion of the coil. The object of making these external 

 insulators funnel-shaped is to prevent surface creeping, due 

 to condensation of moisture on them, by placing paraffin oil 

 or insulating liquid in the funnel-shaped cavity. When this 



