Thermometer, Maximum & Minimum

Quick Answer: The Thermometer, Maximum & Minimum is a U-shaped liquid-in-glass thermometer with two scales and two small indices that are pushed by the liquid and left behind at the highest and lowest temperatures reached. A maximum and minimum thermometer shows a day’s or night’s extremes without anyone having to be there when they happen.

How Six’s Design Records Extremes

The design goes back to James Six, who described it in 1782. A U-shaped tube joins two bulbs. Liquid in one bulb expands and contracts with temperature and pushes a thread of a second, denser liquid around the U-bend. On each side a small steel index rests on top of that thread. As the thread advances it pushes one index ahead of it; when it retreats, the index stays put, held by a light spring or friction. One index therefore marks the highest point reached and the other the lowest.

Because the thread moves in opposite directions in the two limbs, the minimum scale reads upside down, with numbers increasing downward. Read each extreme at the lower end of its index, the end that touched the thread, not at the thread itself. The thread’s present position on either scale shows the current temperature. After recording, the indices are drawn back onto the thread with a small magnet or a push-button, depending on the model, ready for the next period.

Traditional instruments used mercury for the U-bend thread; given the phase-out of mercury thermometers, ask about the fillings if your institution is mercury-free. For a present-moment room reading, the wall thermometer is simpler; for a digital view of two places at once, see the indoor-outdoor thermometer.

Specifications

Applications

• Daily maximum and minimum records at a school weather station
• Greenhouse and polyhouse monitoring for frost or overheating
• Checking overnight extremes in reagent stores, animal houses and seed stores
• Studying how the daily temperature range changes across seasons in geography and biology

Care & Handling

• Mount it upright in shade, ideally inside a ventilated screen when used outdoors.
• Reset both indices at the same time each day, straight after recording.
• Move the indices gently; jarring the instrument can shift them or split the thread.
• Transport and store it upright.

Why Choose LabEquip

Schools with weather stations, nurseries and store managers buy max-min thermometers for unattended records. LabEquip lists this model in General Lab Products; ask about the reset type and fillings through the contact page.

Frequently Asked Questions

Why does the minimum scale run upside down?

On the minimum side the thread moves up as the temperature falls, so the scale has to increase downward. Read the minimum at the lower end of its index.

Where exactly should I read each value?

Read the maximum and the minimum at the end of each index that was in contact with the thread, which is the lower end. Read the present temperature at the top of the thread on either side; the two should agree.

How often should it be reset?

Once per recording period, usually daily at a fixed time. Each reading then shows the extremes of the previous 24 hours.

Why do the two sides show slightly different current temperatures?

A small difference comes from manufacturing tolerance or a slight gap in the thread. A large difference usually means the thread has separated and needs rejoining before readings can be trusted.

Can it be used outdoors?

Yes, if it is shaded from direct sun and protected from rain, ideally in a ventilated screen at a standard height. In direct sunlight it records the heating of the instrument, not the air.

What happens if I forget to reset it?

The indices stay at the extremes since the last reset, so the next reading covers a longer period. Record the date of the last reset with every reading.

Last Updated: September 2026