Pickups and Records: Questions About Pull in a Music Studio

Strange Pulls at the Studio

1Every Saturday, the Northside Youth Music Studio lets teens record songs with help from sound technician Ms. Adaeze Thorvaldsen. One morning, eighth graders Callum Ibarra and Wren Okafor were setting up an electric guitar. Under its steel strings sits a pickup: a row of small magnets wrapped in a coil of very thin copper wire. When a steel string vibrates above the magnets, it makes a tiny changing current in the coil, and the amplifier turns that current into sound.

2Callum had raised the pickup until it almost touched the strings, hoping for a louder sound. Instead, the low notes sounded wobbly and faded quickly. Ms. Thorvaldsen explained that the magnets pull on the steel strings, and a pickup set too close can pull hard enough to disturb how a string vibrates. She said the studio's guitars play best when the pull on a string stays below about 12 mN.

3Later, Wren slid an old vinyl record out of its paper sleeve to play it. The studio's air was dry that day. Bits of dust and lint jumped up onto the record from about 1 cm away. After she wiped the record with a dry wool cloth, dust jumped onto it from about 3 cm away. When she brushed it with the studio's special brush, which has thin carbon fibers that let charge flow away, almost no dust jumped at all.

4The pair asked questions and then ran two tests. For Test 1, they taped a 10 cm piece of steel guitar string to a small wooden block on a digital scale. They clamped a pickup magnet above the string and changed the gap between them. The magnet pulled up on the string, so the scale reading dropped, and they turned each drop into a force in millinewtons (mN). For Test 2, Ms. Thorvaldsen helped them wind 50 turns of wire around a steel bolt to make an electromagnet. They held it 2 mm from the same string piece and used a low-voltage power supply to change the current through the coil. They kept the same string, bolt and gap for every run of Test 2. Each value in the tables is the average of three readings.