TRANSACTIONS OF THE SECTIONS. 43 



dium, and which consists essentially of an endless cord passing with friction 

 through buttons supported on elastic strings. By altering the relation between the 

 friction and the elasticity of different parts, it can be made to exhibit very com- 

 pletely the phenomena observed when an electromotive force is made to act : — (1) 

 between the ends of a metal wire ; (2) through an electrolytic liquid, when it 

 illustrates the convection of electricity by the cathion and the polarization of the 

 electrode ; (3) in an accumulator with perfectly insulating dielectric, when it shows 

 the polarization of the dielectric, the displacement of electricity in the dii'ection of 

 the force, the tension along the lines of force, occasional possible disruptive dis- 

 charge, and consequent possible internal charge ; (4) across a dielectric which is 

 homogeneous, but has a slight conducting power, showing in this case a continuous 

 ordinary conduction-current, in addition to the variations of electric displacement ; 

 (5) across a non-homogeneous or stratified dielectric, in which a " residual charge " 

 is possible. If made of proper materials, the model would exhibit this residual 

 charge quantitatively as well as qualitatively ; and, in fact, the investio:ation " On 

 the Theory of a Composite Dielectric " (in arts. 328-330 of Maxwell's ' Electricity') 

 would apply to it with little modification. It further illustrates incidentally the 

 action of a voltaic cell and of a submarine cable. 



On a Mechanical Illustration of Thermoelectric Plienomena. 

 By Oliver J. Lodge, B.Sc. 



The model which illustrates metallic conduction in the preceding communica- 

 tion is supposed to be modified, so that all the buttons execute very rapid isochro- 

 nous simple harmonic motions, sliding to and fro on the cord. The rate of cooling 

 of a body placed in an enclosure at absolute zero is then seen to be proportional to 

 the absolute temperature of the body, and to depend on its specific electrical resist- 

 ance. The electric condition of tourmaline is explained by an hypothesis as to the 

 nature of its internal structure ; and the amount of heat generated by an electric 

 current passing through a metallic conductor is deduced in accordance with Joule's 

 law. An hypothesis is then started as to the nature of the internal actions at a 

 junction either of two different metals at the same temperature or of two parts of 

 the same metal at different temperatures ; and, on the strength of this hypothesis, 

 electromotive force produced by contact, the Peltier efiect, and Thomson's electric 

 convection of heat are all illustrated. The exact laws which have been experi- 

 mentally established for these efiects may possibly be deducible from considerations 

 founded on the model ; but this has not yet been properly done*. 



On the Protection of Buildings from Lightning. 

 By Professor J. Clerk Maxwell, F.B.S. 



Most of those who have given directions for the construction of lightning- 

 conductors have paid gi'eat attention to the upper and lower extremities of the 

 conductor. They recommend that the upper extremity of the conductor should 

 extend somewhat above the highest part of the building to be protected, and that 

 it should terminate in a sharp point, and that the lower extremity should be car- 

 ried as far as possible into the conducting strata of the ground, so as to " make " 

 what telegraph engineers call " a good earth." 



The electrical effect of such an arrangement is to tap, as it were, the gathering 

 charge, by facilitating a quiet discharge between the atmospheric accumulation and 

 the earth. The erection of the conductor will cause a somewhat greater number 

 of discharges to occur at the place than would have occurred if it had not been 

 erected, but each of these discharges will be smaller than those which woidd have 

 occurred without the conductor. It is probable, also, that fewer discharges will 

 occur in the region surrounding the conductor. It appears to me that these arrange- 

 ments are calculated rather for the benefit of the surrounding country, and for the 



* These two papers are published, with some additions, in the Phil. Mag. ser. 5, vol. ii. 

 pp. 353 and 524. 



4* 



