Condenser too small, burned out, badly insulated (see other pages on this subject).
Battery too small—too high internal resistance or wires leading from battery to coil too small—for ordinary coil work, distance of, perhaps, ten feet, use No. 10 to 12 B & S flexible lamp cord or solid wire. Ruhmkorff coils require plenty of current to produce large sparks.
——————————————————————+————————-+——————-+————————-+——————-+——————— | ½ | 1 | 2 | 6 | 12 | inch | inch | inches | inches| inches ——————————————————————+————————-+——————-+————————-+——————-+——————— Foil sheets | 5½ × 4 | 6 × 4 | 6 × 6 | 10 × 5| 12 × 8 Number | 40 | 40 | 60 | 60 | 60 Paper sheets | 6½ × 5 | 9 × 5 | 8½ × 7 | 12 × 7| 14 × 10 Number | 60 | 60 | 80 | 80 | 80 Core length | 5 | 7 | 9 | 12 | 19 Core diameter | ⅝ | ¾ | 1″ | 1⅛ | 1½ Primary size B & S | 16 | 14 | 14 | 12 | 10 Secondary size B & S. | 36 | 36 | 36 | 36 | 38 Core wire size B W G. | 22 | 22 | 22 | 22 | 22 Quantity in pounds of | | | | | secondary wire | ¾ | 1¼ | 2½ | 7 | 12 Layers of primary | 3 | 3 | 2 | 2 | 2 Area of paper, sq. in.| 2,000 | 2,700 | 4,800 | 6,600 |11,000 Area of foil, sq. in. | 880 | 960 | 2,100 | 3,000 | 5,760 ——————————————————————+————————-+——————-+————————-+——————-+———————
As it is not always convenient to procure paper and foil in set sizes, the area of material needed for condensers is also given. The above table is approximate. It represents data collected from the best modern practice. The gauge above given for copper wire is that of Brown & Sharpe, and is used throughout these pages.
The main points of difference between coils for electrotherapeutics and Ruhmkorff coils is that the former are devoid of condensers, are rarely insulated to a high degree, and are arranged for current strength regulation. The modes of regulation are many, briefly the principal are: (_a_) In coils with independent circuit breakers, sliding both core and primary coil out of the secondary together or independently. (_b_) Moving a metal tube over or off the primary coil or core or both. Many combinations of these methods are practised. Attempts have been made to regulate battery current by rheostat, but it is not feasible, except in large stationary outfits. Cheap medical coils are wound with bare wire, with layers of thread between adjacent turns, or even only bedding the wire turns in paraffined paper. It is not intended to convey the idea that winding bare wire coils is a makeshift; far from it. This method is being very generally adopted in telephone work. But it requires special and delicate machinery, and is unsuited to amateur work, where slight differences of cost or labor are insignificant. Others for specific purposes consist of a primary coil only. The best and most complete made are so arranged that independent secondary coils of different sized wires can be used with the one primary, being readily slipped on or off as required. There is another scheme of regulation, where the coil is wound in sections and these sections cut in or out by means of a switch, but it is not desirable.