Lighting rod insulators. To contain electricity, you need insulators. Before electricity was made, there was the Big Electricity to deal with: lightning! So, the earliest glass insulators were LRIs (Lightning Rod Insulators), used to insulate lightning rods and cables from structures. They date to the 1840s or earlier, and were made in glass until about the turn of the century.
Antique Clear Hemingray Electrical Insulators on Etsy, $5.00
DIY - Reuse Glass Insulators for Succulent Planting
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