138 Axial Galvanometer. 
force indicated will vary with the degree of insertion of the bar. 
When its legs are just within the helices, the action is slight ; as 
they descend further, the action increases, until they reach a 
point about two thirds the way down the helices, when the ac- 
tion is at its maximum. By raising and lowering the shelf 7, 
the action may be varied accordingly, and when the bar is so far 
inserted as to give the maximum of effect, it should be left thus 
far inserted when the greatest degree of sensitiveness is required. 
It will be found that from the first insertion of the legs of the 
bar within the helices, to their entire insertion up to the bend 9, 
the force is continually exerted to draw down the bar; and in 
the experiments performed with a bar 10 inches long, the force 
exerted by five pairs Grove’s battery was equal to two pounds. 
As the point of greatest action is neither in the centre, nor at the 
extremities of the helix, but somewhere between them, I find 
that the position of this point varies with the length of the bars 
employed, and has different relations in differently proportioned 
helices. This instrument is not offered asa sensitive galvano- 
scope, but is calculated to measure the force of currents when 
large batteries are used, or any number of small batteries joined 
as acompound battery. It affords at once a valuable instrument 
for determining the properties of helices of various sizes and 
lengths, of magnets, and of all those relations and proportions 
between battery surface, size of iron for magnets, and number 
of convolutions and size of wire, which must be determined before 
the economy of magnetic power can be settled. It would require 
a vast expenditure of time and material to settle the above points, 
for it is obvious that the magnetism of the bar does not depend 
upon the quantity of electricity which the battery is capable of 
generating, but upon the quantity circulating in the wire around 
the magnet, as well also as upon other conditions. For instance, 
if the wire employed in these experiments be wound after the 
method of Prof. Henry in separate strands, the five pairs Grove’s 
will manifest but very little action upon the magnet or inclosed 
bar. And if the same wire is one continuous piece, as it is actu- 
ally used, and the same surface exposed in the five pairs be con- 
verted into one pair, the action will then also be very slight.* 
This is one of the most important principles to be regarded in all 
* The calculations made, and advanced by several European philosophers, upon 
the amount of zinc consumed, cannot in view of the above, be regarded as afford- 
ing a test of the inapplicability of electro-magnetism as a mechanical agent. 
