Prof. Ritchie on Electricity and Magnetism. 455 
of white marble, and 1310 feet in that of Portland stone *. 
In a pendent x times the necessary strength 7 will be doubled 
in the vth part of the above intervals. 
Explanation of Plate IV. 
Fig. 1. Gothic Pendent. 
Fig. 2. Pendents of Uniform Strength: a, of Portland 
Stone; 4, of White Marble. 
Edinburgh, January 16, 1836. 
LXXVII. Experimental and Physical Researches in Electri- 
city and Magnetism. By the Rev. Witiiam Ritcute, 
LL.D., F.R.S., Professor of Natural Philosophy in the Royal 
Institution and in the University of London.t 
1 AS soon as the magneto-electric spark and shock were 
obtained, it must have been observed that the size of 
the spark increased with the length of the coil employed, and 
afterwards diminished till it at length disappeared. The 
physiological effects are also exceedingly feeble with a short 
coil, and continue to increase by increasing the length of the 
wire long after the spark has attained its maximum bright- 
ness. In experimental research, and particularly in public 
lectures, it is very convenient to obtain both effects from the 
same magnet and revolving lifter. This is easily and expedi- 
tiously accomplished by the following arrangement, which will 
be understood by simply inspecting the annexed figure. 
AB is the hollow axis, C D the solid axis passing through 
the former, metallic contact being prevented by a cylinder of 
wood. B is the disc of 
copper or platina dipping 
into the mercury contained 
in the cell F, and G the 
star or point dipping into 
the cell H. ‘Two copper 
wires having their ends 
formed into a close spiral 
by rolling them round a 
thick wire are soldered to 
the hollowand solid axis at 
Band C. The revolving lifter of soft iron is considerably longer 
* If xo denote the logarithm of 2 upon the scale in question, and M the 
modulus of the common system, we shall have 
Whence these numbers are computed. 
+ Communicated by the Author. 
