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1. The relation between a magnet’s power and the inten- 
sity of the current passing through its helices. 
2. The effect of the distribution and number of its spires 
of wire. 
3. That of the unexcited portion of the magnetic circuit. 
4. That of the material, iron, or hard steel ; and— 
5. The influence of the length and diameter of the magnet. 
The present communication referred to the first only; the 
others being reserved for future occasions. 
He then described his apparatus. 
The electro-magnet consisted of two cylinders of soft iron, 
2 inches diameter, and 12 long, fixed on an iron base, 6 inches 
asunder. On these were placed helices of the same length, 
containing 638 turns of lapped copper wire, No. 12; the mag- 
net weighs 26 lbs., and its keeper 7. The attractive force is 
measured by a weighing machine, composed of two levers, the 
lower of which acts as a steel-yard. The ratio of its leverage 
is nearly 60, and it can be depended on to 34, of the load 
weighed. 
The currents were measured by a tangent Rheometer, 
whose construction was detailed ; its needle’s length is 3 of the 
diameter of its circular conductor, and its law was verified by 
the voltameter up to a deflection of 72°. 
They were equalized by a new Rheostat, which he had for- 
merly exhibited to the Academy, and of which he gave some 
further details. The variable wire is palladium, and it is sur- 
rounded by water, both to cool it and to give a measure of its 
temperature. Unless a correction be applied for that change 
of resistance, which depends on heat, he thought no rheostat 
measures are tobe trusted. As approximations to the resistance 
of this wire, he gave that, assuming his unit of current to be 
that which decomposes a grain of water in five minutes, the 
intensity of a Grove’s cell is 47.28 inches of it. He gave ano- 
ther in terms of the electrolytic intensity of water, stating, 
however, the uncertainty which attaches to this latter mea- 
