Engineering Expert Witness Blog

     I remember one experiment particularly well where I took a big steel nail and coiled wire around it.  When I hooked a battery up to the wires, as shown in Figure 1 below, electric current flowed from the battery through the wire coil.  This set up a magnetic field in the steel nail, thereby creating an electromagnet.  My electromagnet was strong enough to pick up paper clips, and I took great pleasure in repeatedly picking them up, then watching them unattach and fall quickly away when the wires were disconnected from the battery.

Figure 1

     Little did I know then that the electromagnet I had created was similar to an important part found within electrical relays used in many industrial control systems.  An example of one of these relays is shown in Figure 2.

Figure 2

     So, what’s in the little plastic cube?  Well, a relay is basically an electric switch, similar to the ones we’ve discussed in the past few weeks, the major difference being that it is not operated directly by human hands.  Rather, it’s operated by an electromagnet.  Let’s see how this works by examining a basic electrical relay, as shown in Figure 3.

 Figure 3

     The diagram in Figure 3 shows a basic electric relay constructed of a steel core with a wire coil wrapped around it, similar to the electromagnet I constructed in my 5^th grade experiment.  If the coil’s wires are not hooked up to a power source, a battery for example, no electric current will flow through it.  When there is no current the coil and steel core are not magnetic.  For purposes of our illustration and in accordance with industrial control parlance, this is said to be this relay’s “normal state.”

     Next to the steel core there is a movable steel armature, a kind of lever, which is attached to a spring.  On one end of the armature is a pivot point, on the other end is a set of electrical switch contacts.  When the relay is in its normal state, the spring’s tension holds the armature against the “normally closed,” or N.C., contact.  If electric current is applied to the wire leading to the pivot point on the armature while in this state, it will be caused to flow on a continuous path through the armature and the N.C. contact, then out through the wire leading from the N.C. contact.  In our illustration, since the armature does not touch the N.O. contact, an air gap is created that prevents electric current from traveling through the contact from the armature.

     Next week we’ll see how these parts come into play within a relay when electric current flows through the coil, turning it into an electromagnet.

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