or ARTS AND SCIENCES. 377 



their difference of tension, the resistance is augmented in projoortion to 

 the tendency to discharge, and thus each stratum is efFectually precluded 

 from the production of sparks within itself. It only remains, therefore, 

 to interpose a sufficient insulation between the successive strata to pre- 

 vent discharges from one to the other, and the instrument becomes 

 secure from the enfeebling or destructive losses incident to the old con- 

 struction of the coil. Such was the theory of the impi-ovement ; and 

 Mr. Ritchie soon devised a contrivance for winding the helix in planes 

 perpendicular to the axis, carrying the wire alternately from the inner 

 to the outer circumference, and from the outer to the inner, and at the 

 same time securing perfect insulation within and between the strata. 



The new construction proved eminently successful, conferring on the 

 coil a tension much greater than had hitherto been attained. By fur- 

 ther improvements in details, Mr. Ritchie has continued to add to its 

 power, so that now, while the best European coil cannot be relied on 

 for a spark of more than five inches, Mr. Ritchie's first-class instrument 

 projects its luminous flash across an interval of fifteen inches, and ex- 

 hibits other electrical phenomena on a scale of corresponding magnitude 

 and splendor. 



Among the subordinate improvements devised by Mr. Ritchie is a 

 new construction of the breakpiece, in which a spring, bearing the pla- 

 tinum plate and pressing it firmly against the " anvil," secures a closer 

 contact than by the ordinary arrangement. The separation is made 

 by the blow from a spring-hammer worked by a small ratchet-wheel. 

 In this way the time of contact is sufficiently prolonged to allow the 

 iron core to be fully magnetized and the electricity to be developed 

 throughout the Avire, which, in a helix of thirty miles, must require an 

 appreciable time. This advantage is of course lost in the automatic 

 interrupter of De la Rive, where the armature is so instantly with- 

 drawn as in a moment to break the current. In Mr? Ritchie's plan, 

 moreover, the manipulation of the instrument is placed entirely under 

 the control of the operator, so that by varying the intervals of interrup- 

 tion he can vary the length and chai'acter of the spark, and by proper 

 adjustment obtain the greatest length of spark of which the apparatus 

 is capable. To prevent the discharge taking place through the primary 

 coil or its core, Mr. Ritchie, as a substitute for the insulating tube, in- 

 terposes a bell-glass closed at the top, and with its lower edge turned 

 outward in a flange. He makes the secondary helix in one or several 



VOL. IV. 48 



