YEEDET’S CONSTANT IN ABSOLUTE UNITS. 
9 
Verification. 
Now in the length of the helix there are 91 windings on the outside layer, and 
the ratio of the length to the difference of the internal and external radii is 
26-34 „ K . . 
i -o about. 
3 47 
assuming that the number of layers per centim. of radius is the same as the number 
of windings per centim. of length (it would really be a little greater, as they fit into each 
other, o 0 o 0 o°o 0 ) 5 we have 
which is sufficiently near the calculated result, viz. w=1028, to show that no large mis- 
take has been made, as, for instance, writing down a log with a wrong index. 
Closer agreement could hardly be expected, as the helix was made for a different pur- 
pose, and no particular pains were taken to wind it uniformly. It is also probable that 
the instrument-maker took more pains to wind the wires of the outside layer, which 
could be seen, close together than those of the inside ones, which were hidden. 
Determination of Areas. 
1st method . — To calculate the strength of a current in a helix from the deflection of a 
needle placed outside it, it is necessary to know 2(A) where A k is the area of any 
winding. 
To find 2(A) the helix was so placed that a vertical plane through its centre, and 
normal to its axis, contained the magnet and mirror when only acted on by the earth’s 
magnetism. In this plane, between the helix and magnet, and so arranged that it could 
he slid along in it, was placed the small dynamometer. The centres of magnet, dyna- 
mometer, and helix were in the same horizontal line. 
The same current was sent in opposite directions through both helix and small 
dynamometer*, and the latter moved till there was no action in the magnet. The dis- 
tances of the centres from the magnet were then found f to he : — 
Centre of helix to magnet 82 - 80 centims. 
„ dynamometer to magnet . . . 18-10 ,, 
2nd method . — From these data a value of the area was obtained. On looking through 
these experiments, however, Professor Maxwell was of opinion that the dynamometer 
used was not large enough to ensure accuracy, so he had the kindness to compare the 
helix with the great B.A. dynamometer at Cambridge. In his experiments the helix 
and dynamometer were placed concentric, and different currents were sent opposite ways 
* The circular hase of the dynamometer was cemented to a square piece of glass, which slid on a little 
wooden stand, furnished with a guide-bar lying in the magnetic meridian. 
t See paragraphs on measurement of horizontal distances and determination of magnetic meridian, pp. 4 & 16. 
MDCCCLXXVII. C 
