Solenoid Valve Coil Polarity

Why does electrical current polarity not matter
with DC coils?

Below is an excerpt from the book Principles and Practice of Electrical Engineering by Alexander Gray. This excerpt answers the question why solenoid valve coil polarity does not matter.

This book was published by McGraw-Hill in 1917. Although the book is old, the principle in question is still as true today as it was in 1917. (The book has become public domain and we would like to thank Google Book Search for making this book available to all of us to read!)

Principles and Practice of Electrical Engineering

Chapter VIII - Solenoids and Electromagnets

Pull of Solenoids. - A solenoid is a conductor wound in the form of a helix. When an electric current is passed round a solenoid a magnetic field is produced, the direction of which may be determined by the corkscrew law, page 5. This field may be represented by lines of force as shown in diagram A, Fig. 44.

If long bar magnets are placed in the solenoid field as shown in diagram B, Fig. 44, then the n pole of magnet x will tend to move in the direction of the lines of force...and be pulled into the solenoid, while the s pole of magnet y will tend to move in a direction opposite to that of the lines of force so that it also tends to move into the solenoid.

If the current in the solenoid is reversed, the magnetic field of the solenoid will reverse and the magnets x and y will be repelled.

If as in diagram C, Fig. 44, soft iron plungers are used instead of bar magnets, then the lines of force produced by the solenoid will pass through the plungers and cause magnetic poles to be induced; north poles will be formed where the lines of force leave the iron and south poles where they enter... The induced polarity of the plungers shown in diagram C is the same as the polarity of the bar magnets in diagram B so that the plungers are pulled into the solenoid.

If the current in the solenoid is now reversed, the magnetic field of the solenoid will reverse but, since the induced polarity of the plungers will also reverse, the direction of the pull on the plungers will be unchanged.