130 
POPULAR SCIENCE REVIEW. 
lamps will at once perceive that it does not differ from them in many 
respects. The two carbon electrodes are vertical, the upper or positive one 
being fixed ; the lower one is attached to the armature of an electro-magnet, 
and is capable of moving up and down vertically within a space of about 
one-tenth or one-eighth of an inch ; it is supported at the required height 
by two springs, which are capable of adjustment by means of a screw, 
and is drawn downwards by the attractive force of the magnet upon the 
armature when it approaches too near the upper electrode ; it requires a 
weight varying from 150 to 4G0 grains to depress the armature, according 
to the degree of tension imparted by the screw to the second spring. The 
apparatus is very solid and durable. 
The development of the marvellous power of the electric induction coil 
is still progressing, and it is impossible at present to say how much further 
it will proceed. It began but a few years ago to excite attention, when 
sparks half an inch in length were obtained by Rhumkorff, in his admir- 
able apparatus, by means of superior insulation and greater length of 
wire ; from that time many investigators have been engaged in extending 
its power : Mr. Hearder, of Plymouth ; Dr. Callan, of Mavnooth ; Messrs. ■ 
Bentley and Ladd, of London, and others, gradually increased its power ' 
of giving sparks to a distance of about five or six inches; and Mr. Ritchie, , 
of Boston, United States, in a large coil which he made for Mr. Gassiott, 
of London, by winding the enormous quantity of fifty miles of fine wire 
on his coil in a peculiar manner, obtained sparks about twelve or fourteen 
inches long ; and even this length of spark has been considerably exceeded 
by Rhumkorff', who has obtained sparks nineteen inches long. Recently, 
however, at the last conversazione of the Royal Society of London, a very 
remarkable coil was exhibited by Mr. Siemens, and excited great admira- 
tion, which has altogether surpassed previous attainments in this direction. 
First, we will take the effects said to have been obtained by it, and then 
briefly describe its construction and peculiarities. On the evening in 
question, with two elements or cells of Bunsen’s battery connected with 
the primary coil, the two wires of the primary coil being connected 
parallel, sparks were obtained twelve inches in length, and, with six 
elements, they were thirteen inches and a half long ; but, by connecting 
the two primary wires as one continuous wire, the sparks were increased 
in length to nineteen inches ; the electricity then escaped in brilliant 
flashes of light through the air from many parts of the coil. On the above 
occasion, the coil was not worked to its full power, in consequence of the 
batteries being hurriedly prepared, and therefore did not give the maxi- 
mum length of sparks. On a previous occasion, the following effects were 
stated to have been obtained : — A single Bunsen’s element gave sparks 8 - 4 
inches long ; two elements, 15‘6 inches; three ditto, 19 inches; four ditto, 
20 - 4 inches ; five ditto, 22 inches ; and six ditto, 23’2 inches. 
In all cases with this coil, the mechanism for making and breaking con- 
tact is worked by a separate battery consisting of Daniell’s elements, and 
the contact at the break is made by a platinum point, dipping into a vessel 
containing mercury amalgam covered with spirits of wine. The plates of 
the Bunsen’s battery for exciting the coil were about ten inches wide. The 
iron core of the coil is 37'4 inches long, and 2 - 3G inches diameter, and 
