JANUARY 4, 1884.] 
so-called ohm-coils that are in current use. 
The latest experiments indicate that the value 
of the British association unit is .9867 ohms ; 
this result having been obtained by Lord Ray- 
leigh by two distinct methods, and by Mr. 
Glazebrook by still another method. But dif- 
ferent observers still differ quite widely in their 
results. : 
The International committee on electrical 
units, which met in November, 1882, in Paris, 
in view of the present unsettled state of the 
case, and the necessity for the speedy adoption 
of a suitable standard, decided that when the 
length of a column of pure mercury of one 
square millimetre section, and having a resist- 
ance of one ohm, shall have been determined 
to within one part in a thousand, the ohm 
shall then be defined as the resistance of such 
a column of pure mercury of the determined 
length; and the different governments repre- 
sented were urged to prosecute experiments 
for the accurate determination of this length. 
For this purpose, among others, an appropria- 
tion of twelve thousand five hundred dollars 
was made by the last Congress of the United 
States. The work on the unit of resistance is 
under the charge of Professor Rowland of the 
Johns Hopkins university ; and the experiments 
are being carried on in Baltimore, both at the 
university and at Clifton Park, two miles from 
the city. Owing to some unexpected delays in 
the construction of necessary apparatus, the 
work that has been undertaken first is the de- 
termination of the specific resistance of mer- 
cury in British association units. This has been 
experimented upon by measuring the resistance 
of columns of pure mercury contained in glass 
tubes of various calibers and lengths, so that. 
the resistances of the columns experimented 
upon range from one to ten British association 
units. The remaining part of the work is the 
determination in ohms of the resistance of the 
British association standard used in this deter- 
mination of the specific resistance of mercury. 
Two principal methods will be employed for 
this purpose. 
First, the resistance will be found by means 
of the mechanical equivalent of heat. The ap- 
paratus used by Professor Rowland, in his well- 
known work on that subject, has been set up 
for this purpose. It is proposed to heat some 
non-conducting liquid, such as alcohol or tur- 
pentine, by means of the heat developed by 
the passage of the current of electricity in a con- 
ductor whose extremities are kept at a known 
difference of potential. The same heating 
will then be produced, under the same circum- 
stances, by purely mechanical means; and the 
SCIENCE. 11 
resistance of the conductor will thus be deter- 
mined directly from the work-equivalent of the 
heat developed in the conductor. 
The second method to be used is that of 
Kirchoff, as modified by Rowland in his deter- 
mination of the ohm in 1876. ‘The instruments 
will, however, be in large part new, and con- 
structed expressly for this research ; so that a 
new set of instrumental constants will be in- 
volved. A third method, the earth-inductor 
method of Weber, will also be used if time 
permits. 
For these experiments it is proposed to use, 
as a source of electricity in the calorimetric 
method, fifty Planté cells charged by a small 
dynamo machine. For measuring large cur- 
rents of electricity an electrodynamometer has 
been constructed, with the Helmholtz arrange- 
ment'of two large coils and a single small 
suspended coil. The diameter of the large coils 
is about one metre: that of the small suspended 
coil is about twenty-five centimetres. There 
are two sets of large coils, —one wound with 
large wire, about no. 8 ; and the other with much 
smaller, about no.15. Thereare also two small 
suspended coils wound to correspond. This ar- 
rangement gives the instrument great power and 
range. ‘The divided circle was made by Fauth 
& Co. expressly for this instrument. Four in- 
duction-coils are to be wound in four parallel 
equidistant grooves, turned on the outside of 
a brass cylinder about one metre in diameter. 
’ These coils will each consist of about two hun- 
dred turns of no. 15 copper wire. This arrange- 
ment will afford great variety in the manner in 
which the several coils may be combined; for 
the inductive action of each coil upon each of 
the others may be taken, giving three simple 
combinations for each coil. 
The trustees of the Johns Hopkins university 
have kindly placed the Clifton House at Pro- 
fessor Rowland’s disposal for the conduct of 
these experiments; and, as it stands in exten- 
sive grounds at a considerable distance from the 
road, it will be peculiarly suitable for delicate 
electrical experiments. Piers have been built 
for the different instruments, and a small steam- 
engine set up for supplying the power necessary 
for running the dynamo machine and the me- 
chanical equivalent of heat apparatus. The 
actual experimentation will be carried on, under 
Professor Rowland’s direction, by A. L. Kim- 
ball, assisted by H. R. Goodnow and Ensign 
Louis Duncan, U.S.N.; the latter having been 
specially detailed for the work by the Navy 
department. 
It is hoped that a satisfactory conclusion 
will be reached by September, 1884. 
