🔍 Key Concepts

• How humidistats work – they react to changes in relative humidity by sensing moisture in the air • Materials that absorb moisture and expand/contract – hygroscopic materials change length or shape when humidity changes • Metals vs non‑metals – think about which listed materials naturally change dimension with moisture

💭 Think About

• Which of these materials is known to be hygroscopic (able to absorb water from the air) and change its length as humidity changes? • Would common structural metals like copper or steel noticeably change dimension when exposed to normal room humidity, or are they mostly unaffected? • Of the four options, which is traditionally used in simple mechanical humidity indicators and older style hygrometers?

✅ Before You Answer

• Verify which material is commonly used in traditional mechanical hygrometers and humidistats • Eliminate materials that are not significantly affected by moisture, especially common metals • Consider which material can be formed into a fine strand or bundle whose length changes can be easily linked to a switch mechanism