Which invention in 1845 significantly impacted hair styling by enabling temporary straightening of curly hair?

The invention that significantly impacted hair styling in 1845 by allowing for the temporary straightening of curly hair was the hot metal comb. This tool was used to apply direct heat to the hair, which would help to alter its texture, providing a method for individuals with curly hair to achieve a straighter appearance temporarily.

The hot metal comb worked by taking advantage of thermal conductivity; when heated, the metal would conduct heat to the hair, making it pliable enough to be straightened. This was a revolutionary step in hairstyling at the time, creating new possibilities for fashion and personal expression.

Hair straightening lotion and the electric curling iron, while important developments in hair styling, either came later or served different functions, such as providing chemical straightening or curling capability rather than a method of temporary straightening with direct heat. Similarly, the permanently waving machine focused on creating long-lasting waves or curls, which is contrary to the goal of straightening curly hair. Thus, the hot metal comb marked a significant advancement specifically aimed at the temporary straightening of hair.