364 
]\fR. E. H. GRIFFITHS OR THE VALUE OF 
cuiTent, a time will come when the loss by radiation, &c., must equal the gain 
experienced by the calorimeter during the time it was below the temperatui-e of the 
surrounding space. The temperature at which the effects of radiation, &c., are elimi¬ 
nated, I term the “ null point,” and an investigation into its position indicates that if q 
be the time that the calorimeter was beneath the external temperature, and time 
required to raise it from its initial temperature to the same number of degrees above, 
that it was previously beneatli, the temperature of the surrounding envelope, then 
the calorimeter would arrive at its null point at a time somewhere between 2^ and L. 
In the Appendix will be found an investigation into the exact position of this point, 
1887-1889.—During these years my whole attention was directed to an effort to 
ascertain the heat developed by the current while the calorimeter was passing from 
its initial temperature to this null point. Attractive as the method appeared, I 
ultimately relinquished it, for I wished to direct my attention as much to the investi¬ 
gation of the changes in the specific heat of water as to the determination of the 
mechanical equivalent, and, although the method is an admirable one for the latter, 
it is not so suitable for the purposes of the foi’iner enquiry. 
My general method of conducting the experiments was the same as that observed 
in 1892 and described in the succeeding pages. From the outset, I had determined 
to vary all the conditions as much as possible, believing that it was only by such 
means that constant sources of error could be detected. The agreement amongst 
individual experiments ta.ken under the same conditions was, if anything, at times 
more marked than in the experiments of 1892 ; nevertheless, when the final reduction 
of the results took place, fatal discrepancies invariably showed themselves. For 
example, the water equivalent, as deduced from the experiments, increased when the 
mass of the water was increased, and experiments conducted with a high electro¬ 
motive force invariably gave too great a value for the time as compared with that 
obtained when a lower electromotive force was used. These discrepancies, as it now 
appears, may be attributed to two causes ;— 
(1.) To insufficient stirring ; 
(2.) To the rise in temperature of the wire, previously referred to. 
Throughout these experiments I was conscious of the error introduced by ignorance 
of the actual temperature of the wire, but it was not until the summer of 1892 that a 
satisfactory method of estimating this difference of temperature was adopted. 
Unfortunately, many of my experiments in past years were conducted with wires 
coated with a thick insulating covering, and it is, therefore, impossible to apply the 
correction ol )tained this 3 mar to the reduction of the earlier results. The observations 
were excellent in themselves, and some hundreds of experiments were performed. 
They all give too high a value of J (almost invariably above 4'2 X 10’’), and are of no 
Aulue except for the experience gained by their means and tlie improvements they 
suggested in the apparatus. 
