TRANSACTIONS OF THE SECTIONS. 13 



The only circumstance that can affect the accuracy of the indications of the in- 

 strument "is this. The jjiu pressing against the thermometric index displaces it a 

 little, and causes it to assume a position about a degree in advance ; but as this 

 pressure is a constant one, the inconvenience is remedied by a slight corresponding 

 shifting of the scale. 



Electricity, Magnetism. 



On the Electric Induction of Mr. Hooper''s Insulated Wires, compared ivith 

 Gutta-Percha Insulated Wires, for Telegrajyhic Cables. Bij William 

 Hooper. 



The author referred to the relation existing between the different properties of 

 insulated wires arising from induction. He showed by an extensive series of ex- 

 periments that an intimate connexion exists between the effects of electrification 

 and electrostatic induction, and that the penetration of electricity into the substance 

 of an insulator, when measured by the residual discharge, is a function of the electro- 

 static capacity, and not simply of resistance. He has also shown that the effects 

 of electrification are increased nearly in the same proportion as the interior induc- 

 tive action is reduced. 



On a new form of Dynamo-Marfnetic Machine. 

 By William Ladd, F.B.A.S. 



Siemens and "Wlieatstone have shown that the residual magnetism left in soft 

 iron, after being under the influence of a batten,^, or permanent steel magnets, can 

 be augmented from the currents generated by itself, bj- merely applj-ing d^Tiamic 

 force to the revolving armature containing a coil of copper wire, the terminals of 

 which are connected with the wire surrounding the electromagnet, but although 

 great effects were produced in the electromag-net, the current itself could only be 

 made available by its partial or total disruption — in the former case diminishing 

 the power of the electromagnet, and in the latter reducing it to its normal condition. 

 The author has constructed a machine, in which the power of the electromagnet is 

 kept up, whilst a separate current, to be applied to any useful purpose, can be drawn 

 oft by means of an independent arrangement. The machine consists chiefly of two 

 plates of ii'on ; to both ends of each plate is fixed a poiiion of a hollow cylinder ; 

 these plates are tlien placed a certain distance apart, and insulated fi'om each other, 

 in such a manner that the cylindrical pieces form two hollow circular passages ; into 

 these spaces two Siemens's armatures are placed. 



The plates are surrounded by coils of stout copper wire connected together, 

 the two tenuinals being brought into connexion with the commutator of the 

 smaller armature, so that each change of polarity hi the armature will augment the 

 power of the electromagnet. When the machine is first made, it is only requisite 

 to pass a current from a small voltaic cell for an instant, to give the iron a polarity, 

 it will then retain a sufficient amount of magnetism for all future work. 



If the armature in connexion with the electromagnet is made to rotate, there 

 will be a ^ery feeble current generated iu it ; this, passing round the electromagnet, 

 will increase its jiower willi every additional impulse. It will thus be seen that 

 the only limit to the power of the machine is the rapidity with which the arma- 

 ture is made to rotate, which is entirely dependent en the amonnt of dynamic force 

 employed; but the great improvement iu this machine is the introduction of the 

 second armature, which, although it takes oft' very powerful currents generated in 

 its wire by the increased magnetism, does not at all interfere with the primary 

 current of the electromagnet. The machine exhibited in the Paris Exhibition 

 measures about 24 in. in length, 12 in. in width, and stands 7 in. high, which, 

 notwithstanding its imperfect proportions, is capable of keeping 56 in. of platinum 

 wire, '01 in. diameter, incandescent, when a small voltameter was placed in circuit 

 would give oft' 250 cubic centimetres of gas per minute ; and in connexion with an 



