ON THE SPECIFIC RESISTANCE OF MERCURY. 
183 
at the Cavendish Laboratory, called F. For tube IV. another of their unit coils, called 
the Flat coil , was used in multiple arc with F. For tube I., F and a five-ohm coil 
were used in multiple arc. The standard coils belonging to the British Association 
have recently been carefully compared with each other by Professor Fleming, who 
has drawn out a chart in which is recorded their variation with temperature, together 
with their resistance in terms of the mean of their resistances at the temperatures at 
which they were originally considered to be correct. The values of F and of the Flat 
coil —both platinum-silver coils—were taken from this chart. The five-ohm coil had 
been compared with the British Association standards by ourselves. It was also of 
platinum-silver, and its temperature coefficient was assumed to be the same as that of 
the others. 
The standard coils were immersed in water whose temperature was observed each 
time a resistance was measured. These temperatures are given in the table. It may 
be worth remarking that the resistances were taken in a different room from that in 
winch the lengths were measured, which accounts for the difference between t and the 
temperature of the standards. The thermometer used to find all the temperatures 
was graduated to fifths, and was corrected by one which had been verified at Kew. 
When one coil only was used to balance the tube, its terminals fitted directly into 
the mercury cups of the bridge, but when two were used in multiple arc their 
terminals were put into larger mercury cups, which were connected with the mercury 
cups of the bridge by short copper connecting pieces of about '00017 ohm resistance. 
All the measurements were repeated with reversed battery currents, in order to 
eliminate thermoelectric disturbance. The readings with battery current each way 
usually agreed very closely, and the mean of the two was adopted. 
It will be observed that the values of B for tube IY. differ by nearly two parts in 
10,000, and that there is a less proportional difference, but still an appreciable one, 
for the other tubes. The greatest actual difference between any two of the values in 
the table for the same tube is '00014 ohm. Some small error is due to neglect of the 
change of resistance of the copper connecting rods and of the bridge wire with tem¬ 
perature. A change of 4° in the temperature of the rods would make a difference 
of about ‘00003 ohm. There is further a probability of error in ascertaining the 
temperature of the standard coil. A difference of y^ 0 in this also introduces a differ¬ 
ence of '00003 ohm in the resistance; and there is not only a probable error of 
perhaps yy in finding the temperature of the water in which the coil is immersed, but 
there is no certainty that the coil follows the water exactly. There is evidence, how¬ 
ever, that the differences in R are partly due to a real difference in the resistance of 
different fillings of the tube—whether owing to microscopic bubbles or to a thin vary- 
mg layer of air between the mercury and the glass, or to what cause, we were unable 
to determine.* 
* A variation in the closeness of contact between mercury and glass amounting to less than one-fifth 
of a wave-length of mean light would account for the difference of resistances in the two fillings of 
tube IV. 
