2 LL&L << 
ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 101 
As to the cause of these changes, we can say but littie. We hope to 
investigate them more completely by the aid of the coils lent by Mr. 
H. A. Taylor and others, and referred to in the 1888 Report; but it 
seems possible that they are due to strains set up in the wire by the 
great contractions and expansions produced by cooling and heating in 
the paraffin in which the coils are embedded. The coil Flat is of a 
different shape from the others, and little or no paraffin has been used in its 
construction. The other coils, F, G, H, are embedded in paraffin in the 
usual way. On cooling down to 0°, this shrinks greatly, and it is quite 
conceivable that this shrinkage may have strained the coils and so caused 
the change. We hope to test this by having coils made free from parafiin 
and investigating with them the effects of repeated heating and cooling. 
The fall of Hand G would be accounted for by a loss of insulation 
causing a slight leak either from the wire to the case or across the surface 
of the paraffin. The insulation resistance for F, G, H, was therefore 
tested and found in each case to be several thousand megohms, while the 
surface of the paraffin which had become dirty with time was scraped, 
but without producing any change in the resistance. A leak, of course, 
would not produce the rise found in F. 
Observations of the coils at 0° have always been unsatisfactory and 
attended with considerable difficulty. This is mainly due, I believe, to 
the fact that the temperature of the room in which the observations have 
been made has usually been above zero, and that heat is conducted into 
the coils by the thick copper connecting-rods. It would seem possible, 
however, that part of the difficulty (See Report of the Committee for 
1888, Table VII.) may have been due to real changes in the resistance 
arising from strains set up by the cooling. 
Tue Lecan Onm STANDARDS. 
The results of observations on the legal ohm standards of the Associa- 
tion are given in the Report for 1886. Experiments made on. these 
between July 1884 and January 1886 showed that while one coil, ©, 100, 
had retained its value unchanged, the other, , 101, had varied. These 
observations have been continued, and the results are shown in the 
following tables, which give the value of each coil as found by direct 
comparison with the standard B.A. units, and its value as given by the 
chart in 1886. 
Taste V.—Results for ¢, 100. 
Tem- 
Standard used in . Value on ae 
pte comparison perature Walve Chart | a 
Feb. 1887 F 16:3 1:00009 1-00008 “00001 
Noy. 1889 G 158 “99997 99996 | ‘00001 
“1 a 14°8 ‘99971 ‘99968 | -00003 
3 3 16:0 “99998 1:00000 | -00002 
Dec. 1889 Flat 14-4 “99962 99959 | :00003 
“4 A | 14:8 -99969 99968 | -00001 
os 7 Lear pal *99925 “99924 ‘00001 
— == 62 | ‘99744 | 99735 | 00009 
_— — | 57 | 99729. | “99720 “00009 
