A better way of connecting a metallic-circuit line to a grounded line is by the use of a special cord circuit involving a repeating coil, such a connection being shown in Fig. 279.  The cord circuit in this case differs in no respect from those already shown except that a repeating coil is associated with it in such a way as to conductively divide the answering side from the calling side.  Obviously, whatever currents come over the line connected with the answering plug will pass through the windings 1 and 2 of this coil and will induce corresponding currents in the windings 3 and 4, which latter currents will pass out over the circuit of the line connected with the calling plug.  When a grounded circuit is connected to a metallic circuit in this manner, no ground is thrown onto the metallic circuit.  The balance of the metallic circuit is, therefore, maintained.

To ground one side of a metallic circuit frequently so unbalances it as to cause it to become noisy, that is, to have currents flowing in it, by induction or from other causes, other than the currents which are supposed to be there for the purpose of conveying speech.

[Illustration:  Fig. 279.  Connection of Dissimilar Lines through Repeating Coil]

Convertible Cord Circuits. The consideration of Fig. 279 brings us to the subject of so-called convertible cord circuits.  Some switchboards, serving a mixture of metallic and grounded lines, are provided with cord circuits which may be converted at will by the operator from the ordinary type shown in Fig. 276 to the type shown in Fig. 279.  The advantage of this will be obvious from the following consideration.  When a call originates on any line, either grounded or metallic, the operator does not know which kind of a line is to be called for.  She, therefore, plugs into this line with any one of her answering plugs and completes the connection in the usual way.  If the call is for the same kind of a circuit as that over which the call originated, she places the converting key in such a position as will connect the conductors of the cord circuit straight through; while if the connection is for a different kind of a line than that on which the call originated she throws the converting key into such a position as to include the repeating coil.  A study of Fig. 280 will show that when the converting key, which is commonly referred to as the repeating-coil key, is in one position, the cord conductors will be cut straight through, the repeating coil being left open in both its windings; and when it is thrown to its other position, the connection between the answering and calling sides of the cord circuit will be severed and the repeating coil inserted so as to bring about the same effects and circuit arrangements as are shown in Fig. 279.

[Illustration:  Fig. 280.  Convertible Cord Circuit]

Cord-Circuit Considerations. Simple Bridging Drop Type. The matter of cord circuits in magneto switchboards is deserving of much attention.  So far as talking requirements are concerned, the ordinary form of cord circuit with a clearing-out drop bridged across the two strands is adequate for nearly all conditions except those where a grounded-and a metallic-circuit line are connected together, in which case the inclusion of a repeating coil has some advantages.