52 EF. H. Hall— Velocity of an Electric Current. 
olumn I gives the readings of the position micrometer. 
Column II, which is the mean of three readings, is the value in 
terms of the microscope micrometer scale of the interval 0250 
rev. occurring between the two successive readings of the posi- 
tion micrometer. Column III is the deviation of these intervals 
from the mean in terms of the microscope micrometer. Column 
IV gives these deviations expressed in millimeters. 
The small residuals in column IV would apparently indicate 
that there are no greater errors to be apprehended from this 
form than from the ordinary construction, and it seems to pos- 
sess very marked advantages for use in all observations which 
are carried throughout the year, and where the micrometer will 
e used on the same objects under the diverse conditions of 
winter and summer. This would seem to be particularly de- 
sirable for researches upon the stellar parallaxes. 
Art. VL—On Boltzman’s Method for Determining the Velocity of 
; an Electric Current; by E. H. Haut. 
In the June number of this Journal is mentioned a note 
current sent by “one or two Daniels’ cells” through his strip 
of gold the velocity 1-2 mm. per second. 
nless I have misunderstood Prof. Boltzman’s note, how- 
ever, there is a fatal objection to the fundamental assumption 
which he makes. I will give very briefly his method of rea- 
soning as I understand it. 
We know, as Prof. Boltzman says, that a conductor bearing 
a current is acted upon by a force tending to move it in a direc- 
tion at right angles to the direction of the magnetic force acting 
upon it. We know, moreover, from the new phenomenon that 
there is at the same time a difference of potential set up be- 
tween points on opposite sides of the conductor, and that the 
electromotive force thus arising is in the same line as the above 
force acting upon the conductor. 
Consider now any particle of electricity in the conductor. 
* March, 1880, p. 200, 
