A plumb line, also called a plummet, is a small weight on the end of a string used to determine a vertical reference line. The name comes from the Latin “plumbum” meaning lead, the material used in early plumb bobs. The “bob” is the weight at the end of the string and gravity pulls it down to ensure the “line” is perfectly vertical. History tells us they were used in ancient Egypt and maybe even earlier. It is a very simple to make and simple to use tool. Part of its attraction is its simplicity; take a piece of string, tie a weight on one end, and hold the other end of the string up in the air.

Sometimes they are more complex. They’re used in most tall structures to create a vertical datum line. A scaffold holds the plumb line with a pointed bob centered over a datum mark on the floor. Then as the building goes up, the scaffolding and the plumb line are extended upward. Lots of tall structures still have the brass datum marks embedded in the floor at the center of the structure.

Sometimes the plumb bob or line might be encased in a tube filled with water to keep it from swinging. If it does swing, it’s called a pendulum and the most common pendulum is the pendulum bob used in a grandfather clock. William Clement invented the “long case” or “grandfather” clock in 1680 using an anchor escapement to transfer the energy of the swinging pendulum to a wheel which turns the hands of the clock. The weight is movable. If the line is shortened, the pendulum swings faster, and if it’s lowered, the pendulum swings more slowly allowing someone to adjust the accuracy of the clock.

Even God uses a plumb line.

“God showed me this vision: My Master was standing beside a wall. In his hand he held a plumb line. God said to me, ‘What do you see, Amos?’ I said, ‘A plumb line.’ Then my master said, ‘Look what I’ve done. I’ve hung a plumb line in the midst of my people Israel. I’ve spared them for the last time. This is it!’” (Amos 7)

The basic use of a plumb line ensures that something is vertical or straight or, in the grandfather clock, accurate. So God uses the plumb line to test his people. He holds up what is vertical and straight and accurate and compares it to his people.

John Wesley was “a man of one book.” He believed the Bible was God’s plumb line. In the Old Testament, God’s plumb line was summarized in Ten Commandments and later in two commandments — love God with all you have and love your neighbor as yourself. In the New Testament, God’s plumb line might be summarized as the “fruits of the Spirit, love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control.”