=Ques. What is the character of the lines of force of a solenoid in which a current is flowing?= Ans. The lines of force must be thought of as closed loops linked with the current. The conductor conveying the current passes through all the loops of force, and these are, so to speak, threaded or slung on the current-line of flow, as in fig. 116. =Ques. What is the distribution of the lines of force?= Ans. The lines of force form continuous closed curves running through the interior of the coil; they issue from one end and enter into the other end of the coil, as shown in fig. 117. =Ques. What are the properties of a solenoid?= Ans. A solenoid has north and south poles, and in fact possesses all the properties of an ordinary permanent magnet, with the important difference that the magnetism is entirely under control. Since a solenoid carrying a current attracts and repels by its extremities the poles of a magnet, two such solenoids will attract and repel each other. [Illustration: FIG. 118.--Magnetic field of a solenoid. This is best observed by cutting a piece of cardboard and fitting it around the solenoid, as shown. If iron filings be sprinkled on the cardboard and a current passed through the solenoid, the character of the field is indicated. With the current in the direction shown, it will be found that wherever small compass needles are placed, the direction in which their north poles turn is along arrows marked on the card. The card only exhibits the field in one of the sectional planes of the coil, but it is obvious that the field is the same for all sectional planes.] =Ques. How does the magnetic strength of a solenoid vary?= Ans. It is proportional to the strength of the electric current passing through it. =Ques. On what, besides the current strength, does the magnetizing power of a solenoid depend?= Ans. _The magnetic effect or the magnetizing power is proportional to the number of turns of wire composing the coil._ =Ques. How may the magnetizing power of a solenoid be increased?= Ans. By inserting in the solenoid an _iron core_ or round bar of soft iron. [Illustration: FIG. 119.--Right hand rule for polarity of a solenoid: If the solenoid be grasped in the right hand, so that the fingers point in the direction in which the current is flowing in the wires, the thumb extended will point in the direction of the north pole.] =Ques. Describe the action of an iron core.= Ans. At first, the presence of an iron core greatly increases the strength of the field; after a time, however, as the strength of the current flowing in the exciting coils is increased, the _conductibility_ of the iron for the lines of force appears to decrease, until a point is eventually reached when the presence of the iron core appears to have no effect in increasing the strength of the field.