Keeb Kwm: Tus balancer muaj keeb kwm ntau tshaj 100 xyoo. Xyoo 1866, German Siemens tau tsim lub tshuab hluav taws xob. Plaub xyoos tom qab, ib tug Canadian, Henry Martinson, patented cov txheej txheem ntsuas, pib kev lag luam. Xyoo 1907, Dr. Franz Lawa...
Nyeem ntxiv